Fix libz and liblzma detection

If libz or liblzma are detected with pkg-config, AC_CHECK_HEADERS must
not be run because the correct CPPFLAGS aren't set. It is actually not
required have separate checks for LIBXML_ZLIB_ENABLED and HAVE_ZLIB_H.
Only check for LIBXML_ZLIB_ENABLED and remove HAVE_ZLIB_H macro.

Fixes bug 764657, bug 787041.
diff --git a/c14n.c b/c14n.c
index a0f2c19..c04ce66 100644
--- a/c14n.c
+++ b/c14n.c
@@ -2089,7 +2089,7 @@
         xmlC14NErrParam("saving doc");
         return (-1);
     }
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     if (compression < 0)
         compression = xmlGetCompressMode();
 #endif