package/gflags: include TARGET_CXXFLAGS to the overloaded CXXFLAGS

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/gflags/gflags.mk

@@ -11,7 +11,8 @@ GFLAGS_LICENSE = BSD-3c
 GFLAGS_LICENSE_FILES = COPYING.txt
 
 ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),)
-GFLAGS_CONF_OPTS = -DBUILD_gflags_LIB=OFF -DCMAKE_CXX_FLAGS=-DNO_THREADS
+GFLAGS_CONF_OPTS = -DBUILD_gflags_LIB=OFF \
+	-DCMAKE_CXX_FLAGS="$(TARGET_CXXFLAGS) -DNO_THREADS"
 endif
 
 $(eval $(cmake-package))