Set OS=android via GYP_DEFINES

Makefile.android

 # 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 80 matching lines @@
 ANDROID_TOOLCHAIN ?= ${TOOLCHAIN_PATH}/${TOOLCHAIN_DIR}
 
 ifeq ($(wildcard $(ANDROID_TOOLCHAIN)),)
   $(error Cannot find Android toolchain in "${ANDROID_TOOLCHAIN}".  Please \
           check that ANDROID_NDK_ROOT and ANDROID_NDK_HOST_ARCH are set    \
           correctly)
 endif
 
 # For mksnapshot host generation.
 DEFINES += host_os=${HOST_OS}
+DEFINES += OS=android
 
 .SECONDEXPANSION:
 $(ANDROID_BUILDS): $(OUTDIR)/Makefile.$$@
         @$(MAKE) -C "$(OUTDIR)" -f Makefile.$@ \
                   CXX="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-g++" \
                   AR="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-ar" \
                   RANLIB="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-ranlib" \
                   CC="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-gcc" \
                   LD="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-ld" \
                   LINK="$(ANDROID_TOOLCHAIN)/bin/${TOOLCHAIN_PREFIX}-g++" \
                   BUILDTYPE=$(shell echo $(subst .,,$(suffix $@)) | \
                               python -c "print raw_input().capitalize()") \
                   builddir="$(shell pwd)/$(OUTDIR)/$@"
 
 # Android GYP file generation targets.
 ANDROID_MAKEFILES = $(addprefix $(OUTDIR)/Makefile.,$(ANDROID_BUILDS))
 $(ANDROID_MAKEFILES):
         GYP_GENERATORS=make-android \
         GYP_DEFINES="${DEFINES}" \
         CC="${ANDROID_TOOLCHAIN}/bin/${TOOLCHAIN_PREFIX}-gcc" \
         CXX="${ANDROID_TOOLCHAIN}/bin/${TOOLCHAIN_PREFIX}-g++" \
         PYTHONPATH="$(shell pwd)/tools/generate_shim_headers:$(shell pwd)/build:$(PYTHONPATH)" \
         build/gyp/gyp --generator-output="${OUTDIR}" build/all.gyp \
                       -Ibuild/standalone.gypi --depth=. -Ibuild/android.gypi \
                       -S$(suffix $(basename $@))$(suffix $@) ${GYPFLAGS}