finally fix libraries test (hope)

makefile

@@ -53,7 +53,7 @@ ifneq (0, $(CPPUNIT_EXISTS))
 $(error You need cppunit installed)
 endif
-LIBXML2_EXISTS = $(shell $(CXX) -o /dev/null -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++ - <<< $$'\#include <libxml/xmlreader.h>\nint main() {\nreturn 0;\n}'; echo $$?)
 ifneq (0, $(LIBXML2_EXISTS))
 $(error You need libxml2 installed)
 endif