diff --git a/package/proj/proj.mk b/package/proj/proj.mk
index ca943141ce..faa7dab19b 100644
--- a/package/proj/proj.mk
+++ b/package/proj/proj.mk
@@ -12,11 +12,15 @@ PROJ_INSTALL_STAGING = YES
 PROJ_DEPENDENCIES = host-pkgconf host-sqlite sqlite
 
 PROJ_CFLAGS = $(TARGET_CFLAGS)
+PROJ_CXXFLAGS = $(TARGET_CXXFLAGS)
 
 ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_68485),y)
 PROJ_CFLAGS += -O0
+PROJ_CXXFLAGS += -O0
 endif
 
-PROJ_CONF_ENV = CFLAGS="$(PROJ_CFLAGS)"
+PROJ_CONF_ENV = \
+	CFLAGS="$(PROJ_CFLAGS)" \
+	CXXFLAGS="$(PROJ_CXXFLAGS)"
 
 $(eval $(autotools-package))