sdl2.m4: Deprecate AM_PATH_SDL2 in favour of PKG_CHECK_MODULES AM_PATH_SDL2 doesn't add much beyond PKG_CHECK_MODULES, and having a special m4 macro for every library that you might depend on scales poorly. The macro does add special support for macOS frameworks, but that feature was broken for around 6 years without anyone noticing (#6141), and is likely to be only rarely useful according to comments on #6141. Resolves: #6140 Signed-off-by: Simon McVittie <smcv@collabora.com>