thunderbird: fix gcc6 build

srcpkgs/thunderbird/template

@@ -22,6 +22,15 @@ makedepends="
  libSM-devel libXScrnSaver-devel libXt-devel libXdamage-devel"
 depends="desktop-file-utils hicolor-icon-theme"
 
+_gccver=$(gcc --version | awk '/^gcc \(GCC\)/ { print $3 }')
+if [ "${_gccver%%.*}" -gt 5 ]; then
+        # Append CFLAGS and CXXFLAGS to set work around code which gcc6 would
+        # otherwise regard as out-of-specification and allow it to produce a
+        # working program.
+        CFLAGS+=" -fno-delete-null-pointer-checks -fno-lifetime-dse -fno-schedule-insns2"
+        CXXFLAGS+=" -fno-delete-null-pointer-checks -fno-lifetime-dse -fno-schedule-insns2"
+fi
+
 pre_configure() {
 	case "$XBPS_TARGET_MACHINE" in
 	*-musl)