[gimp] configure: check presence of "gegl:matting-levin" with `gegl --exists`.

[Jehan <jehanp src gnome org>]

commit 9560a653c5c1554cd4e9e79afbe08283698f2ec6
Author: Jehan <jehan girinstud io>
Date:   Mon Sep 17 16:40:00 2018 +0200

    configure: check presence of "gegl:matting-levin" with `gegl --exists`.
    
    This is a runtime dependency. If absent, we simply won't have access to
    the alternative Matting Levin engine in the foreground selection tool.
    
    If we don't add a test in configure, this may be easily forgotten. I
    created the `gegl --exists` feature specifically for this kind of
    checks, so let's check operation existence.

 INSTALL.in   |  1 +
 configure.ac | 13 +++++++++++++
 2 files changed, 14 insertions(+)
---
diff --git a/INSTALL.in b/INSTALL.in
index d15bddd489..93cf39e526 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -195,6 +195,7 @@ header files installed.
      xdg-email for sending emails
      sendmail for sending emails if --with-sendmail enabled
      gdb or lldb for our new bug-reporting dialog
+     "gegl:matting-levin" GEGL operation for alternative matting engine
 
 Please make sure you don't have any old GTK+-2.x, jpeg, etc. libraries
 lying around on your system, otherwise configure may fail to find the
diff --git a/configure.ac b/configure.ac
index 7949cc419e..ed6cd74652 100644
--- a/configure.ac
+++ b/configure.ac
@@ -978,6 +978,18 @@ AM_CONDITIONAL(HAVE_EXCHNDL, test "x$ac_cv_lib_exchndl_ExcHndlSetLogFileNameA" =
 
 AC_CHECK_HEADERS([execinfo.h])
 
+#########################
+# Check for Matting Levin
+#########################
+
+AC_MSG_CHECKING([for matting engine Levin])
+if $GEGL --exists "gegl:matting-levin"; then
+  have_matting_levin=yes
+else
+  have_matting_levin='no (missing GEGL operation "gegl:matting-levin")'
+fi
+AC_MSG_RESULT($have_matting_levin)
+
 ##########################################
 # Check for some special functions we need
 ##########################################
@@ -2934,6 +2946,7 @@ Extra Binaries:
   gimp-console:              $enable_gimp_console
 
 Optional Features:
+  Levin matting engine:      $have_matting_levin
   Language selection:        $have_iso_codes
   Vector icons:              $enable_vector_icons
   Dr. Mingw (Win32):         $enable_drmingw