mesa: Fix xcb-dri2 link flags leaking into LIBS.
mesa: Fix xcb-dri2 link flags leaking into LIBS.

Fixes the build of builtin_compiler on my 32-bit build where xcb-dri2
is in a custom prefix but the custom prefix flags weren't available.
It shouldn't have been in LIBS anyway.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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