osmesa: Allow building standalone in all three channel widths
osmesa: Allow building standalone in all three channel widths

autoconf had been designating the 8 bit libOSMesa as the default
standalone osmesa, but the Makefile expected it to be linked to libGL.
Fix up the osmesa Makefile so that it allows any of the combinations of
standalone and channel width to be built.

Fixes bug #21980.
(cherry picked from commit 7441dcd90b01df8351026af8bbb50e11bb86071a)

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