build: Make gtk-doc optional

This is similar to what's done in glib.

autogen.sh

@@ -16,8 +16,8 @@ set -e
 
 mkdir -p m4
 
-GTKDOCIZE=$(which gtkdocize 2>/dev/null)
-if test -z $GTKDOCIZE; then
+GTKDOCIZE=$(which gtkdocize 2>/dev/null || true)
+if test -z "$GTKDOCIZE"; then
         echo "You don't have gtk-doc installed, and thus won't be able to generate the documentation."
         rm -f gtk-doc.make
         cat > gtk-doc.make <<EOF
@@ -25,7 +25,7 @@ EXTRA_DIST =
 CLEANFILES =
 EOF
 else
-        gtkdocize || exit $?
+        gtkdocize
 fi
 
 cd $olddir

configure.ac

@@ -112,7 +112,14 @@ OSTREE_FEATURES="$OSTREE_FEATURES +gpgme"
 
 LIBARCHIVE_DEPENDENCY="libarchive >= 2.8.0"
 
+# check for gtk-doc
+m4_ifdef([GTK_DOC_CHECK], [
 GTK_DOC_CHECK([1.15], [--flavour no-tmpl])
+],[
+enable_gtk_doc=no
+AM_CONDITIONAL([ENABLE_GTK_DOC], false)
+])
+
 AC_PATH_PROG([XSLTPROC], [xsltproc])
 
 AC_ARG_WITH(libarchive,