Fix libxml2 makefile check for CXX=clang

makefile

@@ -75,7 +75,7 @@ ifneq (0, $(CPPUNIT_EXISTS))
 $(error You need cppunit installed)
 endif
-LIBXML2_EXISTS = $(shell $(CXX) -o /dev/null -I/usr/include/libxml2/ -lxml2 -xc++ - <<< $$'\#include <libxml/xmlreader.h>\nint main() {\nreturn 0;\n}'; echo $$?)
+LIBXML2_EXISTS = $(shell $(CXX) -o /dev/null -I/usr/include/libxml2/ -lxml2 -xc++ -std=c++11 - <<< $$'\#include <libxml/xmlreader.h>\nint main() {\nreturn 0;\n}'; echo $$?)
 ifneq (0, $(LIBXML2_EXISTS))
 $(error You need libxml2 installed)
 endif