Revert "scons: Refuse to use LLVM 2.7 for now."
Revert "scons: Refuse to use LLVM 2.7 for now."

This reverts commit 44703217f73095c4946e06057e399e447b252576.

http://www.llvm.org/bugs/show_bug.cgi?id=6429 has been fixed now. Things
appear to be working reasonably so far.

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