build: fix HAS_GI_DOCGEN never defined error

When building without --enable-bindings-glib configure reports this error:

...
checking for help2man... true
checking that generated files are newer than configure... done
configure: error: conditional "HAS_GI_DOCGEN" was never defined. Usually this means the macro was only invoked conditionally.
make: *** [Makefile:440: config.status] Error 1

Move the initialization of HAS_GI_DOCGEN outside the conditional with_bindings_glib section so it is always initialized.

---
 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index cbe9e13..1ac1002 100644
@@ -267,12 +267,12 @@ then
 		AC_MSG_ERROR([glib-mkenums not found - needed to build GLib bindings]))
 
 	AC_CHECK_PROG([has_gi_docgen], [gi-docgen], [true], [false])
-	AM_CONDITIONAL([HAS_GI_DOCGEN], [test "x$has_gi_docgen" = xtrue])
 	if test "x$has_gi_docgen" = xfalse
 	then
 		AC_MSG_NOTICE([gi-docgen not found - GLib documentation cannot be generated])
 	fi
 fi
+AM_CONDITIONAL([HAS_GI_DOCGEN], [test "x$has_gi_docgen" = xtrue])
 
 # GObject-introspection
 found_introspection=no

2.39.2