Bug 1243037 - part 2 - export RUSTC from configure; r=mshal

Because of how subshells and exporting variables works with msys bash,
we need to explicitly re-export everything interesting for things
executed downstream from configure.

configure.in

@@ -9198,6 +9198,7 @@ export MOZ_APP_NAME
 export MOZ_APP_REMOTINGNAME
 export DONT_POPULATE_VIRTUALENV=1
 export PYTHON
+export RUSTC
 export MOZILLA_CENTRAL_PATH=$_topsrcdir
 export STLPORT_CPPFLAGS
 export STLPORT_LIBS