configure.ac: add new option --enable-bundled-mypaint-brushes

because a simple platform dependency in gimpcoreconfig.c doesn't do
the trick (there can be both manual builds and bundled builds on
windows and macos). Use an AC_DEFINE() instead.

Also, make sure the system gimprc and its manpage are generated with
the correct values.

app/config/gimpcoreconfig.c

@@ -262,7 +262,7 @@ gimp_core_config_class_init (GimpCoreConfigClass *klass)
                          GIMP_CONFIG_PARAM_RESTART);
   g_free (path);
 
-#if defined G_OS_WIN32 || defined PLATFORM_OSX
+#ifdef ENABLE_BUNDLED_MYPAINT_BRUSHES
   mypaint_brushes = g_build_filename ("${gimp_installation_dir}",
                                       "share", "mypaint-data",
                                       "1.0", "brushes", NULL);

configure.ac

@@ -1667,10 +1667,25 @@ PKG_CHECK_MODULES(LIBMYPAINT, libmypaint >= libmypaint_required_version,,
 PKG_CHECK_MODULES(MYPAINT_BRUSHES, mypaint-brushes-1.0,,
                   [add_deps_error([mypaint-brushes-1.0])])
 
-# We need an autoconf variable for man page construction.
-mypaint_brushes_dir=`$PKG_CONFIG --variable=brushesdir mypaint-brushes-1.0`
+AC_MSG_CHECKING([whether to treat MyPaint brushes as bundled])
+AC_ARG_ENABLE(bundled-mypaint-brushes,
+              [  --enable-bundled-mypaint-brushes
+                          treat MyPaint brushes as bundled in the install prefix
+                          (default=no)],,
+              enable_bundled_mypaint_brushes=no)
+
+if test "x$enable_bundled_mypaint_brushes" != xno; then
+  AC_DEFINE(ENABLE_BUNDLED_MYPAINT_BRUSHES, 1,
+            [Define to 1 to if MyPaint brushes are bundled with GIMP])
+  mypaint_brushes_dir='${gimp_installation_dir}/share/mypaint-data/1.0/brushes'
+else
+  mypaint_brushes_dir=`$PKG_CONFIG --variable=brushesdir mypaint-brushes-1.0`
+fi
+
+AC_MSG_RESULT([$enable_bundled_mypaint_brushes])
 AC_SUBST(mypaint_brushes_dir)
 
+
 ##################
 # Check for webkit
 ##################
@@ -2731,6 +2746,7 @@ Optional Features:
   Language selection:      $have_iso_codes
   Vector icons:            $enable_vector_icons
   Dr. Mingw (Win32):       $enable_drmingw
+  Bundled MyPaint Brushes: $enable_bundled_mypaint_brushes (at $mypaint_brushes_dir)
 
 Optional Plug-Ins:
   Ascii Art:               $have_libaa