[gimp] configure: fix LUA test.

[Jehan <jehanp src gnome org>]

commit 2791328e416168437b6e2cdeeeeef24a7018001e
Author: Jehan <jehan girinstud io>
Date:   Thu Sep 12 12:57:52 2019 +0200

    configure: fix LUA test.
    
    Bug discovered by Simon. See #3921.
    The logics of the whole test for binding interpreters may still change
    (or even disappear), but for now, at least make it work as expected.

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index a351bad3d5..2a5113a903 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2390,7 +2390,7 @@ WARNING: luajit not found.
 else
   with_lua="yes"
   AC_CHECK_PROGS(LUA, luajit, no)
-  if test "x$GJS" = xno; then
+  if test "x$LUA" = xno; then
     required_deps="$required_deps
     - luajit not found.
         *** Please install the Lua interpreter luajit.