[android] Configure building android with ninja and clang.

build/gyp_v8

 #!/usr/bin/env python
 #
 # 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
@@ skipping 112 matching lines @@
   # to enfore syntax checking.
   syntax_check = os.environ.get('V8_GYP_SYNTAX_CHECK')
   if syntax_check and int(syntax_check):
     args.append('--check')
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Generate for the architectures supported on the given platform.
   gyp_args = list(args)
-  gyp_generators = os.environ.get('GYP_GENERATORS')
+  gyp_generators = os.environ.get('GYP_GENERATORS', '')
   if platform.system() == 'Linux' and gyp_generators != 'ninja':
     # Work around for crbug.com/331475.
     for f in glob.glob(os.path.join(v8_root, 'out', 'Makefile.*')):
       os.unlink(f)
     # --generator-output defines where the Makefile goes.
     gyp_args.append('--generator-output=out')
     # -Goutput_dir defines where the build output goes, relative to the
     # Makefile. Set it to . so that the build output doesn't end up in out/out.
     gyp_args.append('-Goutput_dir=.')
+
+  gyp_defines = os.environ.get('GYP_DEFINES', '')
+
+  # Automatically turn on crosscompile support for platforms that need it.
+  if all(('ninja' in gyp_generators,
+          'OS=android' in gyp_defines,
+          'GYP_CROSSCOMPILE' not in os.environ)):
+    os.environ['GYP_CROSSCOMPILE'] = '1'
+
   run_gyp(gyp_args)