Fix: sdl2-config would always be detected as present

The presence of sdl2-config is used go determine whether to look for
sdl2 first, or just sdl1. However, when sdl2-config is *not* present,
`which` returns an empty string. Due to lack of quoting, this produces
`[ -x ]`, rather than `[ -x "" ]` and it turns out the former actually
has a succesful exit status for some reason.

This was not a problem when just running configure, because it would
then just fail to detect sdl2 and fall back to sdl1. However, when
passing `--with-sdl` (without specifying a version), this would only
attempt to detect sdl2, even when sdl2-config was not present, but sdl1
is.

Adding quotes makes the check work as intended.

Remove: [OSX] Old fullscreen subdriver for pre 10.7 systems.

Since the move to C++11, building for pre 10.7 is not possible with the native
Apple tools. Also, due to bitrot, the file doesn't even compile anymore. While
this could be fixed, it shows that this subdriver is basically never used anymore.

Remove: [OSX] QuickDraw video subdriver used for OSX versions up to 10.4.

Since the C++11 move, getting OpenTTD to compile and run for anything
below 10.7 basically requires building a custom compiler and libc++.
Also, the QuickDraw subdriver crashes on more modern OSX version. While this
is fixable, keeping the driver around is probably pointless.

Fix: [MINGW32] Fix launch on Windows 7 x64

Prior to this commit, the following is observed when running openttd.exe
that was compiled using mingw32 on x86_64:

> The procedure entry point ScriptBreak could not be located in the
dynamic link library GDI32.dll

The MSDN docs at the below link state that modules using Uniscribe must
first link against usp10 before gdi32 to avoid this issue:

https://msdn.microsoft.com/en-us/library/windows/desktop/dd319118(v=vs.85).aspx

Signed-off-by: Joe Stringer <joe@wand.net.nz>

Fix: Allow git worktree in version detection; drop svn, hg.

If $ROOT_DIR is a linked working tree from git-worktree(1), then
$ROOT_DIR/.git is a regular file instead of a directory. Allow this
when deciding whether to use git to detect OpenTTD's version.

Drop checks for svn and hg in config.lib, because findversion.sh
hasn't used svn nor hg since 192770e6da.

Add: [Win32] GDI engine for font glyph rendering as a replacement for including FreeType.

Building with FreeType is still possible and will take precedence over the GDI renderer, but
the project files don't include FreeType anymore by default. Combining GDI rendering with ICU
text layout is untested.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

Remove: DOS support

In 10 years there was no active development on DOS. Although it
turned out to still work, the FPS was very bad. There is little
interest in the current community to look into this.

Further more, we like to switch to c++11 functions for threads,
which are not implemented by DJGPP, the only current compiler
for DOS.

Additionally, DOS is the only platform which does not support
networking. It is the reason we have tons of #ifdefs to support
disabling networking.

By removing DOS support, we can both use c++11 functions for threads,
and remove all the code related to disabling network. Sadly, this
means we have to see DOS go.

Of course, if you feel up for the task, simply revert this commit,
and implement stub c++11 functions for threads and stub functions
for networking. We are more than happy to accept such Pull Request.

Codechange: ICU_SORT is in reality ICU_I18N (according to their CMake files)

By naming it in a different way, things get a bit confusing.
Especially if we are switching to CMake, which autodetects these
things, we need to use the name the authors of ICU gave it; not
our interpertation of that name.

Codechange: ICU_LAYOUT is in reality ICU_LX (according to their CMake files)

By naming it in a different way, things get a bit confusing.
Especially if we are switching to CMake, which autodetects these
things, we need to use the name the authors of ICU gave it; not
our interpertation of that name.

Codechange: liblzma is called liblzma, how ever strange that might be

It is the only library we use that calls itself with 'lib' in the
name. This might be confusing, but with the arrival of cmake a lot
of these things are automated. And detection will find 'liblzma',
not 'lzma', like with 'lzo', 'zlib', ..

Remove: libtimidity support (NOT timidity support)

libtimidity was introduced with the support for PSP. PSP has been
dropped almost a year ago, but this music driver was not. This
corrects that oversight.

timidity (via extmidi) still works fine. This purely removes the
libtimidity support, which was only really available for PSP.

Remove: BeOS support (deprecated by Haiku)

In 10 years there is no commit to change how BeOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), BeOS is no longer
support. SDL2 suggests to use Haiku instead of BeOS.

Remove: MorphOS / AmigaOS support

In 10 years there is no commit to change how MorphOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), MorphOS is no longer
support. There is an SDL2 port for MorphOS, but it is not maintained
by upstream SDL2, and nobody can currently test it out.

If anyone wants to re-add MorphOS, please do (revert this patch,
fix the problems, and create a Pull Request). If you need any help
doing so, let us know! It is not that we don't like MorphOS, it is
that we don't have anyone fixing the problems :(

Add: [OSX] Text layout using the native CoreText API.

By default, the native API will be used instead of ICU, but if ICU is
forced in using configure, it will take precedence.

Codechange: detect native Apple clang correctly in config.lib #6773

Fixes: 00c16032569d943e4fac5c5b6218dfce47479716

Codechange: Ensure that -lfreetype is always last when linking statically

Fixes weird bug with MinGW

Fix e614357: Ask the compiler who it is, instead of using symlinks (#6727)

This fixes #6723

Change: [Build] prioritize cc-build over build (based on patch by woutershep2) (#6716)

This closes #6414

Change: [Build] use flags set by env at the end, not at the start (based on patch by adf88) (#6715)

This closes #6433