configure: error out when building opencl without LLVM
configure: error out when building opencl without LLVM

Cc: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>
Cc: "10.1 10.2" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>

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