I have run into two problems trying to build an RPM from the source tarball per the README-rpm instructions on a freshly-installed CentOS 6.4 i686 box.
The first one I diagnosed and worked around: In the "cd Test ; make" step in the spec file, at least one of the programs in Test does a #include SDL/SDL_gfxPrimitives.h, which fails because there is no "SDL" directory in the include path at that point. I had to install SDL_gfx from the source tarball build so that there would be an SDL/SDL_gfxPrimitives.h in /usr/local/include for it to find.
In other words, the SDL_gfx RPM cannot bootstrap itself. It depends on having an existing installation in a default location.
The second I've simply run out of time trying to fix. This command:
install -m755 -d /root/rpmbuild/BUILDROOT/SDL_gfx-2.0.25-1.i386/usr/share/SDL_gfx-demos
results in a huge "cp" command (20 or so lines) that fails because apparently there is no Test/m4 directory. I don't know why that is the case, and haven't got time right now to figure it out.