Bug 1349694 - only build gkrust-gtest when we build libxul-gtest; r=chmanchester

To improve build times, we've decided to only build libxul-gtest when
it's specifically requested, i.e. via `mach gtest` or similar.  However,
our build setup specifies that gkrust-gtest, the Rust code specifically
for libxul-gtest, is built during normal builds.  This library takes a
significant amount of time to compile and is generally not needed.

This patch changes the gkrust-gtest library to only be visible under the
same conditions that libxul-gtest is built, thus speeding up normal
builds.

toolkit/library/gtest/rust/Makefile.in

@@ -0,0 +1,29 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# This file looks quite similar to toolkit/library/gtest/Makefile.in.
+# We only want to build gkrust-gtest when we are building libxul-gtest.
+
+# Enforce that the clean/distclean rules removes everything that needs
+# to be removed from this directory.
+ifneq (,$(filter clean distclean,$(MAKECMDGOALS)))
+LINK_GTEST_DURING_COMPILE = 1
+endif
+
+# Don't build gkrust-gtest during MOZ_PROFILE_GENERATE, it doesn't get
+# used during profiling anyway.
+ifdef MOZ_PROFILE_GENERATE
+LINK_GTEST_DURING_COMPILE =
+endif
+
+ifndef LINK_GTEST_DURING_COMPILE
+# Force to not include backend.mk unless LINK_GTEST_DURING_COMPILE is set.
+# Not including backend.mk makes traversing this directory do nothing.
+STANDALONE_MAKEFILE = 1
+
+else
+
+include $(topsrcdir)/config/config.mk
+
+endif