Added support for detecting system libxml2.

configure.ac

@@ -15,8 +15,15 @@ AC_ARG_WITH([libxml2],
 
 if test "x$with_libxml2" != xno; then
   PKG_CHECK_MODULES([libxml2], [libxml-2.0 >= 2.7],
-                    [with_libxml2=yes],
+                    [with_libxml2=yes], [dummy=1])
+  if test "x$with_libxml2" != yes; then
+    old_CFLAGS=$CFLAGS
+    CFLAGS="-I /usr/include/libxml2"
+    AC_CHECK_HEADER([libxml/parser.h],
+                    [with_libxml2=yes; libxml2_CFLAGS="-I /usr/include/libxml2"; libxml2_LIBS="-lxml2"],
                     [AC_MSG_ERROR([couldn't find libxml2])])
+    CFLAGS=$old_CFLAGS
+  fi
 else
   AC_CHECK_HEADER(expat.h, [], [AC_MSG_ERROR([couldn't find expat headers; expat required])])
 fi