Bug 1320738 - export MOZ_PGO for js/src subconfigure; r=gps

Environment variables for configure don't get automatically propagated
down to js/src on Windows because mingw. So we have to take extra steps.

--HG--
extra : amend_source : fc123e7439ebbf97884eeb56af8409c7b3294b21

old-configure.in

@@ -5101,6 +5101,7 @@ export HOST_CFLAGS
 export HOST_CPPFLAGS
 export HOST_CXXFLAGS
 export HOST_LDFLAGS
+export MOZ_PGO
 
 if ! test -e js; then
     mkdir js