Add a makefile option for wasm prototype.

Adding wasm=on when invoking make will build with the wasm prototype, Ex:

make x64.debug wasm=on V=1

BUG=None
TEST=manual
R=titzer@chromium.org,ncbray@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1293073004

Cr-Commit-Position: refs/heads/master@{#30236}

Makefile

@@ -216,6 +216,12 @@ ifeq ($(arm_test_noprobe), on)
   GYPFLAGS += -Darm_test_noprobe=on
 endif
 
+# Optionally enable wasm prototype.
+# Assume you've placed a link to v8-native-prototype in third_party/wasm.
+ifeq ($(wasm), on)
+  GYPFLAGS += -Dv8_wasm=1
+endif
+
 # ----------------- available targets: --------------------
 # - "grokdump": rebuilds heap constants lists used by grokdump
 # - any arch listed in ARCHES (see below)