configure.ac, icons: check for the "gegl" binary

and use the found executable instead of hardcoding "gegl".

configure.ac

@@ -628,6 +628,11 @@ else
 fi
 AC_MSG_RESULT($have_glib_2_43)
 
+AC_PATH_PROG(GEGL, gegl, no)
+if test "x$GEGL" = xno; then
+  AC_MSG_ERROR([Could not find gegl in your PATH])
+fi
+
 
 # Check for bind_textdomain_codeset, including -lintl if GLib brings it in.
 gimp_save_LIBS=$LIBS

icons/Symbolic-Inverted/Makefile.am

@@ -20,7 +20,8 @@ include $(top_srcdir)/icons/Symbolic/icon-list.mk
 
 %.png:
 	mkdir -p `dirname $@`;
-	gegl -x "<gegl> \
+	$(GEGL) -x \
+	"<gegl> \
 	  <node operation='gegl:invert-linear'></node> \
 	  <node operation='gegl:load'> \
 	    <params> \