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 <kenneth@whitecape.org>