autoconf: Fix build of dri symbols test to not manually link expat.
autoconf: Fix build of dri symbols test to not manually link expat.

AC_CHECK_LIB has this nasty behavior, like the cflags tests, of
automatically putting the tested value into the global LIBS on
success.  This caused -lexpat to end up in LIBS, but without the
--with-expat dir, so my 32-bit build on a 64 system using expat from a
custom prefix could only find the system expat and fail to link on the
one current consumer of the LIBS variable: the dri driver test link.

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

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