configure: check for -lgbm

We use the library unconditionally, however not check it in
configure phase. Let's produce an error there and not at
compilation phase.
And while at it, drop -ldl from LDFLAGS too. I suspect it's
coming from the same source because in the gbm pkg-config file
private libs are to be linked with dl. But not only we are not a
gbm private library, we don't use dl*() anywhere.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

configure.ac

@@ -83,6 +83,7 @@ LIBDRM_REQUIRED=2.4.50
 
 PKG_CHECK_MODULES([LIBDRM], [libdrm >= $LIBDRM_REQUIRED])
 PKG_CHECK_MODULES([EPOXY], [epoxy])
+PKG_CHECK_MODULES([GBM], [gbm])
 
 AC_SUBST([DEFINES])
 AC_CONFIG_FILES([

src/Makefile.am

@@ -1,5 +1,6 @@
 SUBDIRS := gallium/auxiliary
-AM_LDFLAGS = -lgbm -lm -ldl \
+AM_LDFLAGS = -lm \
+	$(GBM_LIBS) \
 	$(EPOXY_LIBS) \
 	$(CODE_COVERAGE_LDFLAGS)
 
@@ -9,6 +10,7 @@ AM_CFLAGS = \
 	$(DEFINES) \
 	$(PIC_FLAGS) \
 	$(LIBDRM_CFLAGS) \
+	$(GBM_CFLAGS) \
 	$(EPOXY_CFLAGS) \
 	$(VISIBILITY_CFLAGS) \
 	$(CODE_COVERAGE_CFLAGS)