package/glibmm: use GLIBMM_CXXFLAGS

Commit 8d8d4ecaf1bc22a89d0f373e9d1655f59f283480 ("package/glibmm: work
around gcc bug 85180") was incorrect reworked by me, and
TARGET_CXXFLAGS was used instead of GLIBMM_CXXFLAGS, causing the
Microblaze workaround to be ineffective. This new commit fixes this
typo.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/glibmm/glibmm.mk

@@ -19,6 +19,6 @@ ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_85180),y)
 GLIBMM_CXXFLAGS += -O0
 endif
 
-GLIBMM_CONF_ENV += CXXFLAGS="$(TARGET_CXXFLAGS)"
+GLIBMM_CONF_ENV += CXXFLAGS="$(GLIBMM_CXXFLAGS)"
 
 $(eval $(autotools-package))