proplib: use GCC atomic builtins if supported.

configure

@@ -340,6 +340,31 @@ echo "CPPFLAGS +=       -I\$(TOPDIR)/lib/portableproplib/prop" >>$CONFIG_MK
 echo "LDFLAGS +=	-lpthread" >>$CONFIG_MK
 echo "STATIC_LIBS +=	-lpthread" >>$CONFIG_MK
 
+#
+# Check for GCC atomic builtins.
+#
+func=atomic
+printf "Checking for GCC atomic builtins ... "
+cat <<EOF >_$func.c
+int main() {
+	volatile unsigned long val = 1;
+	__sync_fetch_and_add(&val, 1);
+	__sync_fetch_and_sub(&val, 1);
+	__sync_add_and_fetch(&val, 1);
+	__sync_sub_and_fetch(&val, 1);
+	return 0;
+}
+EOF
+if $XCC _$func.c -o _$func 2>/dev/null; then
+	echo yes.
+	HAVE_ATOMICS=1
+else
+	echo no.
+fi
+rm -f _$func.c _$func
+if test -n "$HAVE_ATOMICS"; then
+	echo "CPPFLAGS +=	-DHAVE_ATOMICS" >> $CONFIG_MK
+fi
 #
 # Check for vasprintf().
 #