autoconf: Add support for osmesa-only builds
autoconf: Add support for osmesa-only builds

Added autoconf support for using OSMesa as the driver instead linking
it to libGL. This is enabled through --with-driver=osmesa.

To differentiate these cases, another option --enable-x11-osmesa is used
to enable or disable building OSMesa when the driver is x11.

benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,KTXSoftware/glsl2agal,zeux/glsl-optimizer,metora/MesaGLSLCompiler,adobe/glsl2agal,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,mcanthony/glsl-optimizer,KTXSoftware/glsl2agal,dellis1972/glsl-optimizer,zeux/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,adobe/glsl2agal,jbarczak/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,adobe/glsl2agal,zeux/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,benaadams/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,tokyovigilante/glsl-optimizer,KTXSoftware/glsl2agal,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,bkaradzic/glsl-optimizer,KTXSoftware/glsl2agal,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,adobe/glsl2agal,mcanthony/glsl-optimizer,adobe/glsl2agal,zz85/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer