primitives: Remove hack to get ALLEGRO_OPENGL, ALLEGRO_DIRECT3D values.
Do not install most internal header files.

I think this practice was mostly carried over from Allegro 4.
By not installing them, we don't have to maintain the list of files
(which was out of date).
Sort ALLEGRO_INCLUDE_ALLEGRO_PLATFORM_FILES.
Delete empty source file lists.
Delete directory name from source file list.
Delete references to header files which no longer exist.
Delete obsolete al388*.h header files.
Delete obsolete ALLEGRO_ASMCAPA_HEADER macros.
Delete unneeded includes of ALLEGRO_INTERNAL_HEADER.
Delete stuff related to constructor attribute (unused).
Delete stuff related to hand-written asm code in A4 (unused).
Delete unnecessary ALLEGRO_CFG_ALLOW_SSE macro.
Delete RETSIGTYPE macro.

We had the return type of a signal handler unconditionally set to `void'
for so long and nobody complained, so just delete it.
Delete RET_VOLATILE macro.
Delete AL_CONST macro.
Delete commented out MAP_FAILED macro.

This was supposed to be defined if somehow not provided in system headers,
but it's only used in an old Linux console port file which is probably not even
used.
Delete some old macros regarding filesystems.

Namely:

ALLEGRO_LFN
OTHER_PATH_SEPARATOR
DEVICE_SEPARATOR
FA_*
O_BINARY
O_TEXT
Delete some obsolete macros about color conversion code.
Delete ALLEGRO_HAVE_LIBPTHREAD macro (assumed where appropriate).
Delete ALLEGRO_MULTITHREADED macro (assumed).