Bug 1283898 - Don't check for rust outside of toolkit builds. r=rillian

Spidermonkey doesn't currently depend on rust code, and this
unblocks enabling rust by default on gecko builds until we
can get the appropriate toolchain hooked up to all of the
SM automation jobs.

The include must be conditional to avoid breaking artifact builds.

MozReview-Commit-ID: 1PmcFvcZLM2

--HG--
extra : rebase_source : 1a22232e064dd253b80ebaa55decfde1ba7e1ea0

build/moz.configure/toolchain.configure

@@ -972,4 +972,3 @@ def is_windows(target):
     return target.kernel == 'WINNT'
 
 include('windows.configure', when=is_windows)
-include('rust.configure')

toolkit/moz.configure

@@ -86,6 +86,12 @@ set_define('MOZ_JACK', jack)
 include('../js/moz.configure')
 
 
+# Rust
+# ==============================================================
+include('../build/moz.configure/rust.configure',
+        when='--enable-compile-environment')
+
+
 # L10N
 # ==============================================================
 option('--with-l10n-base', nargs=1, env='L10NBASEDIR',