build: disable gtkdocize in autoreconf

We have a custom modified gtk-doc.make file in our sources, which
disables the gtkdoc-rebase on local install step, so we don't want
autoreconf to copy a different one when we're bootstrapping.

autogen.sh

@@ -14,7 +14,7 @@ PKG_NAME=ModemManager
 }
 
 (cd $srcdir;
-    autoreconf --force --install --verbose
+    GTKDOCIZE="true" autoreconf --force --install --verbose
 )
 
 if test -z "$NOCONFIGURE"; then