build: Allow autogen-eration on systems without GTK-doc

The autogen.sh script fails if it cannot run gtkdocize. The absence of
this command should not cause cairo builds to fail, it should only
prevent the build of its documentation.

Fixes:
*** No GTK-Doc found, please install it ***

on systems without gtkdoc tools.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=50356

autogen.sh

@@ -15,16 +15,16 @@ fi
 
 GTKDOCIZE=`which gtkdocize`
 if test -z $GTKDOCIZE; then
-        echo "*** No GTK-Doc found, please install it ***"
-        exit 1
+        echo "*** No GTK-Doc found, documentation won't be generated ***"
+else
+        gtkdocize || exit $?
 fi
 
 # create dummy */Makefile.am.features and ChangeLog to make automake happy
 > boilerplate/Makefile.am.features
 > src/Makefile.am.features
 touch ChangeLog
 
-gtkdocize || exit $?
 autoreconf --force --install --verbose || exit $?
 
 cd $ORIGDIR