[gimp] configure.ac: fix libbacktrace test

[Ell <ell src gnome org>]

commit 02457753469a7f0f73a1093d252853a201e6fd3d
Author: Ell <ell_se yahoo com>
Date:   Sat Sep 22 22:45:47 2018 -0400

    configure.ac: fix libbacktrace test
    
    ... added in last commit.

 configure.ac | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/configure.ac b/configure.ac
index 6cff17fa28..1d3bfd63ca 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1799,6 +1799,8 @@ AC_ARG_WITH(libbacktrace, [  --without-libbacktrace       build without libbackt
 
 have_libbacktrace=no
 if test "x$with_libbacktrace" != xno; then
+  have_libbacktrace=yes
+
   gimp_save_LIBS=$LIBS
   LIBS="$LIBS -lbacktrace"