Use -I not -L for includes.

* configure.in (XML_INCLUDEDIR): Use -I not -L for includes.
diff --git a/configure.in b/configure.in
index f3f5efa..159aceb 100644
--- a/configure.in
+++ b/configure.in
@@ -29,7 +29,7 @@
 AC_CHECK_HEADERS(stdarg.h sys/stat.h sys/types.h time.h zlib.h)
 
 XML_LIBDIR='-L${libdir}'
-XML_INCLUDEDIR='-L${includedir}'
+XML_INCLUDEDIR='-I${includedir}'
 XML_LIBS="-lxml $Z_LIBS"
 
 AC_SUBST(XML_LIBDIR)