automake: Honor GL_LIB for gallium libgl-xlib
automake: Honor GL_LIB for gallium libgl-xlib

Use "@GL_LIB@" in src/gallium/targets/libgl-xlib/Makefile.am to produce
the library name specified by the configure --with-gl-lib-name option.

Reviewed-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

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