configure: Disable xvmc by default
configure: Disable xvmc by default

The xvmc unit tests are failing on r300g and r600g.

Reviewed-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>

mcanthony/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer