GYP: don't build V8 twice

common.gypi

@@ -3,6 +3,7 @@
     'visibility%': 'hidden',         # V8's visibility setting
     'target_arch%': 'ia32',          # set v8's target architecture
     'host_arch%': 'ia32',            # set v8's host architecture
+    'want_separate_host_toolset': 0, # V8 should not build target and host
     'library%': 'static_library',    # allow override to 'shared_library' for DLL/.so builds
     'component%': 'static_library',  # NB. these names match with what V8 expects
     'msvs_multi_core_compile': '0',  # we do enable multicore compiles, but not using the V8 way