presence: Makefile - try to use pkg-config if xml2-config is not found

(cherry picked from commit 5ce9690245e0a075742c7cb8eac1786622c976f4)

src/modules/presence/Makefile

@@ -7,6 +7,12 @@ NAME=presence.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 \