defining an alternate process function that will be compiled to mmx
and sse by gcc, coupled with runtime cpu detection at class init time
to determine which variant to use.
* configure.ac: make USE_SSE available to makefiles.
* gegl/Makefile.am: install gegl-cpuaccel.h
* gegl/gegl-chant.h: added logic to detect various SIMD compile modes,
generating different .o 's based on compiler flags.
* gegl/gegl-plugin.h: include gegl-cpuaccel.h
* operations/Makefile-operations.am: added separete recipe for
compiling .c files that contains the string GEGL_SIMD.
* operations/common/brightness-contrast.c: 
* operations/common/invert.c: 
* operations/generated/other-blend.rb:
* operations/generated/svg-12-porter-duff.rb: The rest, updated to
specify their SIMD code using GEGL_SIMD.

svn path=/trunk/; revision=2477