autoconf: Improve help text for demos and DRI drivers
autoconf: Improve help text for demos and DRI drivers

Changed the text for the default demos to be "auto" instead of "yes"
since that didn't make much sense in this context. Added an example
value for the --with-dri-drivers option as suggested by Brian.

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