[gxml] With valadoc not installed avoid to set Documentation enable

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 0829fd0b83941efe85ec45e6c4900d76ca0c5b29
Author: Daniel Espinosa <esodan gmail com>
Date:   Sun Mar 12 12:38:14 2017 -0600

    With valadoc not installed avoid to set Documentation enable

 configure.ac |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 16cc9c4..1b0a817 100644
--- a/configure.ac
+++ b/configure.ac
@@ -156,9 +156,10 @@ AC_ARG_ENABLE(docs,
         [docs=$enableval], [docs="yes"])
 
 have_valadoc=no
-if test "x$VALADOC" = "xno" -a "x$docs" = "xno"; then
+if test "x$VALADOC" = "xno" -o "x$docs" = "xno"; then
        have_valadoc="no"
-       else
+       docs="no"
+else
        have_valadoc="yes"
 fi
 AM_CONDITIONAL([HAVE_VALADOC], [test x$have_valadoc = xyes])