[gxml] Fixed --enable-gtk-docs alone. It requires --enable-valadoc

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 6d56cfd4b9964844a009f70aa686a19d1e2f57b8
Author: Daniel Espinosa <esodan gmail com>
Date:   Sun May 3 09:01:07 2015 -0500

    Fixed --enable-gtk-docs alone. It requires --enable-valadoc

 configure.ac |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 745eaa0..216587f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -175,7 +175,11 @@ AC_ARG_ENABLE([gtk-docs],
 
 have_gtkdoc=no
 if test x$enable_gtk_docs = xyes; then
-  PKG_CHECK_MODULES([GTKDOC], gtk-doc)
+  if test x$have_valadoc = xyes; then
+    PKG_CHECK_MODULES([GTKDOC], gtk-doc)
+  else
+    AC_MSG_ERROR([You should require valadoc enabled to generate gtk-doc documentation. Use 
--enable-valadoc])
+  fi
 fi
 
 AC_ARG_ENABLE([gir-docs],