Bug 1257468 - Add --enable-project=js when calling configure for js/src from subconfigure.py. r=gps

subconfigure.py shortcuts js/src/configure to avoid a msys transition
breaking the environment. But js/src/configure passes an extra
--enable-project=js that subconfigure.py currently doesn't pass.

And because in some cases, MOZ_BUILD_APP is set in the environment
(notably, in OSX universal builds, through client.mk), we need to
unset it before calling the subconfigure.

build/subconfigure.py

@@ -314,6 +314,7 @@ def run(objdir):
             command = [
                 sys.executable,
                 os.path.join(os.path.dirname(__file__), '..', 'configure.py'),
+                '--enable-project=js',
             ]
             data['env']['OLD_CONFIGURE'] = os.path.join(
                 os.path.dirname(configure), 'old-configure')

old-configure.in

@@ -7943,6 +7943,7 @@ export MOZILLA_CENTRAL_PATH=$_topsrcdir
 export STLPORT_CPPFLAGS
 export STLPORT_LIBS
 export JS_STANDALONE=no
+unset MOZ_BUILD_APP
 export DIST
 export MOZ_LINKER
 export ZLIB_IN_MOZGLUE