in SDL 1.3 (revision 5508 from SVN), the method used to calculate the
bits per pixel from a “int format” differ between “SDL_ListModes” (which
always uses the “SDL_BITSPERPIXEL” macro) and “SDL_PixelFormatEnumTo-
Masks” (which uses either “SDL_BITSPERPIXEL” or “SDL_BYTESPERPIXEL * 8”,
depending on the value of “SDL_BYTESPERPIXEL”).

Because the values are later compared in “SDL_ListModes” this may lead
to some valid video modes not being returned. In my case the only mode
returned by “SDL_GetNumDisplayModes” was dismissed and NULL was
returned. (This led to the calling application sticking its head in the
sand.)

The attached patch copies the method used within “SDL_PixelFormatEnumTo-
Masks” to “SDL_ListModes”. This solved the problem for me though I don't
fully understand the method used by “SDL_PixelFormatEnumToMasks”.

I *think* the patch below works correctly: sdl-config gets the -march
flag and SDL builds fine. Perhaps there's a better way to get the
-march flag into both CFLAGS and SDL_FLAGS w/o dragging along more
cruft, but adding it to BASE_CFLAGS seemed the best bet to my eyes.

I did some digging, and I think I've found a better solution. By
including some additional .m4 macros in the acinclude directory
(http://www.gnu.org/software/autoconf-archive/ax_check_compiler_flags.html,
http://www.gnu.org/software/autoconf-archive/ax_gcc_archflag.html and
http://www.gnu.org/software/autoconf-archive/ax_gcc_x86_cpuid.html)
then using the patch below (same as previous patch + one line change
to configure.in), SDL will build correctly out of the box.

The arch can then be overridden by --with-gcc-arch=<arch> (most will
probably want to build with i686 there for general distribution) or
--without-gcc-arch, to disable setting the arch all together.

On my Atom netbook, I get -march=PentiumPro by default, which works
great, as does --with-gcc-arch=i686.

Fixed building SDL test programs

That's actually okay, since gcc has the builtin atomic intrinsics. :)

This allows all SDL code to take advantage of the atomic intrinsics on Windows, but doesn't cause applications just including SDL.h to pull in windows.h