test: do not clobber CFLAGS

Signed-off-by: Rémi Denis-Courmont <remid@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

test/Makefile.am

 AM_CPPFLAGS = -I$(top_srcdir)/include
-CFLAGS = $(X11_CFLAGS)
+AM_CFLAGS = $(X11_CFLAGS)
 dlclose_LDADD = $(DLOPEN_LIBS) $(X11_LIBS)
 
 TESTS = dlclose