configure, libgimpwidgets: customizable default color profile directory

We have this problem with flatpak that the common directory
("/usr/share/color/icc") is not visible from inside the sandbox. With
proper permissions, it is actually accessible, yet the host's /usr is
mounted at /run/host/usr/.
Since sandbox systems are getting more common, we need to make this path
customizable. I am therefore adding a --with-icc-directory configure
option to change the default color profile folder at build time.
See: https://github.com/flathub/org.gimp.GIMP/issues/15

configure.ac

@@ -2341,6 +2341,30 @@ NOTE: if you plan on packaging GIMP for distribution, it is recommended
 "
 fi
 
+#########################
+# Default ICC directory #
+#########################
+
+# This is necessary because some Unix systems may have a different
+# standard path for color profiles. And in particular, sandbox builds
+# might mount the host system at a different root. This is for
+# instance the case of flatpak which mount the host root at /run/host/.
+if test "x$platform_osx" = "xno" &&
+   test "x$platform_win32" = "xno"; then
+  AC_MSG_CHECKING([default directory for color profiles])
+  AC_ARG_WITH(icc-directory,
+              [  --with-icc-directory    Path to default color profiles for this system],,
+              [with_icc_directory="/usr/share/color/icc"])
+
+  if test "x$with_icc_directory" = "x"; then
+    with_icc_directory="/usr/share/color/icc"
+  fi
+
+  AC_DEFINE_UNQUOTED(COLOR_PROFILE_DIRECTORY,
+                     "$with_icc_directory",
+                     [The path to default color profiles for this system])
+fi
+
 ######################################
 # Checks for gtk-doc and docbook-tools
 ######################################
@@ -2827,6 +2851,7 @@ Optional Features:
   Vector icons:            $enable_vector_icons
   Dr. Mingw (Win32):       $enable_drmingw
   Bundled MyPaint Brushes: $enable_bundled_mypaint_brushes (at $mypaint_brushes_dir)
+  Default ICC directory:   $with_icc_directory
 
 Optional Plug-Ins:
   Ascii Art:               $have_libaa

libgimpwidgets/gimpcolorprofilechooserdialog.c

@@ -295,7 +295,7 @@ gimp_color_profile_chooser_dialog_add_shortcut (GimpColorProfileChooserDialog *d
       }
 
     if (! folder_set)
-      add_shortcut (dialog, "/usr/share/color/icc");
+      add_shortcut (dialog, COLOR_PROFILE_DIRECTORY);
   }
 #endif
 }