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

@@ -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

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