Revert of Remove wasm compile time option and enable wasm behind a runtime flag.

Makefile

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 202 matching lines @@
 # arm_test_noprobe=on
 # With this flag set, by default v8 will only use features implied
 # by the compiler (no probe). This is done by modifying the default
 # values of enable_armv7, enable_vfp3, enable_32dregs and enable_neon.
 # Modifying these flags when launching v8 will enable the probing for
 # the specified values.
 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)
 # - any mode listed in MODES
 # - every combination <arch>.<mode>, e.g. "ia32.release"
 # - "native": current host's architecture, release mode
 # - any of the above with .check appended, e.g. "ia32.release.check"
 # - "android": cross-compile for Android/ARM
 # - "nacl" : cross-compile for Native Client (ia32 and x64)
 # - default (no target specified): build all DEFAULT_ARCHES and MODES
@@ skipping 257 matching lines @@
 # We need to manually set the stack limit here, to work around bugs in
 # gmake-3.81 and global-5.7.1 on recent 64-bit Linux systems.
 GPATH GRTAGS GSYMS GTAGS: gtags.files $(shell cat gtags.files 2> /dev/null)
         @bash -c 'ulimit -s 10240 && GTAGSFORCECPP=yes gtags -i -q -f $<'
 
 gtags.clean:
         rm -f gtags.files GPATH GRTAGS GSYMS GTAGS
 
 dependencies builddeps:
         $(error Use 'gclient sync' instead)