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.