gxml test fix (#4560)

Formula/gxml.rb

@@ -48,12 +48,13 @@ class Gxml < Formula
         return 0;
       }
     EOS
-    ENV.libxml2
+    libxml2 = Formula["libxml2"]
     gettext = Formula["gettext"]
     glib = Formula["glib"]
     libgee = Formula["libgee"]
     flags = %W[
       -I#{gettext.opt_include}
+      -I#{libxml2.opt_include}/libxml2
       -I#{glib.opt_include}/glib-2.0
       -I#{glib.opt_lib}/glib-2.0/include
       -I#{include}/gxml-0.6
@@ -62,6 +63,7 @@ class Gxml < Formula
       -L#{gettext.opt_lib}
       -L#{glib.opt_lib}
       -L#{libgee.opt_lib}
+      -L#{libxml2.opt_lib}
       -L#{lib}
       -lgee-0.8
       -lgio-2.0