DOCS: Note that we need SDL2
BUILD: Remove duplicate AC_SUBST()s for FreeType2
BUILD: Move SDL2 checking into ax_check_sdl2.m4
BUILD: Move libfaad checking into ax_check_faad.m4
BUILD: Move libmad checking into ax_check_mad.m4
BUILD: Move libogg/vorbisfile checking into ax_check_ogg.m4/vorbis.m4
BUILD: Move libxvidcore checking into ax_check_xvid.m4
BUILD: Move zlib checking into ax_check_zlib.m4
BUILD: Move iconv() into ax_check_iconv.m4
BUILD: Move OpenAL checking into ax_check_al.m4
BUILD: Move freetype2.m4 to ax_check_freetype2.m4
BUILD: Modify AX_CHECK_GL to include our Mac OS X hack
BUILD: Add a minimum version number to the boost check
BUILD: Group the library checks more usefully
BUILD: Arrange C/CXX flag construction more intelligently