rls: Makefile - use pkg-config if xml2-config is not found

- GH #2199

src/modules/rls/Makefile

@@ -10,6 +10,12 @@ NAME=rls.so
 
 ifeq ($(CROSS_COMPILE),)
 XML2CFG=$(shell which xml2-config)
+ifeq ($(XML2CFG),)
+XML2CFG=$(shell \
+	if pkg-config --exists libxml-2.0; then \
+		echo 'pkg-config libxml-2.0'; \
+	fi)
+endif
 ifeq ($(OS), darwin)
 ICU_BUILDER = $(shell \
 	if pkg-config --exists icu-uc; then \