This patch makes mesa use the ASM_API variable (src/mesa/sources)

configs:

ASM_API is defined for DRI config files as well, because it is used in libGL.
Note: linux-dri-x86-64 and freebsd-dri-amd64 contained ASM_API as part of
ASM_SOURCES. I moved it on its own assuming it was just forgotten there, 
is this ok ?

src/mesa/sources:

move dispatch.c to GLAPI_SOURCES (no need to actually move the file)
this fixes the mess with building dispatch.c in the DRI case,
as discussed in https://bugs.freedesktop.org/show_bug.cgi?id=3899

src/glx/x11/Makefile:

use GLAPI_OBJECTS from src/mesa/sources instead of adding files manually


It also makes two small corrections:
src/mesa/Makefile: mesa.a -> libmesa.a
src/mesa/sources : CORE_SOURCES for src/mesa/Makefile.mgw