Fix testsuite build on OS X.

There's no linker flag of this name in the normal clang, presumably
beause the linker feature doesn't exist.

configure.ac

@@ -52,6 +52,8 @@ AC_SYS_LARGEFILE
 # uintptr_t to a void *") by default.  Kill that.
 XORG_TESTSET_CFLAG(CWARNFLAGS, [-Wno-int-conversion])
 
+has_znow=yes
+
 case $host_os in
     mingw*)
         build_egl=no
@@ -72,6 +74,7 @@ case $host_os in
         build_egl=no
         build_glx=yes
         build_wgl=no
+        has_znow=no
         EPOXY_LINK_LIBS=""
         ;;
     *)
@@ -101,6 +104,7 @@ if test x$build_wgl = xyes; then
     AC_DEFINE([BUILD_WGL], [1], [build WGL tests])
 fi
 
+AM_CONDITIONAL(HAS_ZNOW, test x$has_znow = xyes)
 
 case $host_os in
     mingw*)

test/Makefile.am

@@ -66,6 +66,10 @@ EGL_LIBS = libegl_common.la
 endif
 
 if BUILD_GLX
+if HAS_ZNOW
+GLX_SHARED_ZNOW = glx_shared_znow
+endif
+
 GLX_TESTS = \
 	glx_beginend \
 	glx_public_api \
@@ -73,7 +77,7 @@ GLX_TESTS = \
 	glx_glxgetprocaddress_nocontext \
 	glx_has_extension_nocontext \
 	glx_static \
-	glx_shared_znow \
+        $(GLX_SHARED_ZNOW) \
 	$()
 
 GLX_LIBS = libglx_common.la