allow caller to override the default output directory for gyp

gyp_skia

 #!/usr/bin/python
 
 # Copyright 2011 The Android Open Source Project
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script is a wrapper which invokes gyp with the correct --depth argument,
 # and supports the automatic regeneration of build files if all.gyp is
 # changed (Linux-only).
 
 import glob
 import os
 import platform
 import shlex
 import sys
 
 script_dir = os.path.abspath(os.path.dirname(__file__))
 
 # Directory within which we can find the gyp source.
 gyp_source_dir = os.path.join(script_dir, 'third_party', 'externals', 'gyp')
 
 # Directory within which we can find most of Skia's gyp configuration files.
 gyp_config_dir = os.path.join(script_dir, 'gyp')
 
+# Allow the user to override the directory where gyp should produce its output
+# Default to the current directory.
+gyp_output_dir = os.environ.get('SKIA_GYP_OUTPUT_DIR', '.')
+
 # Ensure we import our current gyp source's module, not any version
 # pre-installed in your PYTHONPATH.
 sys.path.insert(0, os.path.join(gyp_source_dir, 'pylib'))
 import gyp
 
 ENVVAR_GYP_GENERATORS = 'GYP_GENERATORS'
 ENVVAR_GYP_GENERATOR_FLAGS = 'GYP_GENERATOR_FLAGS'
 
 
 def additional_include_files(args=[]):
@@ skipping 90 matching lines @@
   if not gyp_file_specified:
     args.append('skia.gyp')
 
   args.extend(['-I' + i for i in additional_include_files(args)])
   args.extend(['--depth', '.'])
 
   # Tell gyp to write the build files into output_dir.
   args.extend(['--generator-output', os.path.abspath(get_output_dir())])
 
   # Tell ninja to write its output into the same directory.
-  args.extend(['-Goutput_dir=.'])
+  args.extend(['-Goutput_dir=%s' % gyp_output_dir])
 
   # By default, we build 'most' instead of 'all' or 'everything'. See skia.gyp.
   args.extend(['-Gdefault_target=most'])
 
   # Fail if any files specified in the project are missing
   if sys.platform.startswith('win'):
     gyp_generator_flags = os.getenv(ENVVAR_GYP_GENERATOR_FLAGS, '')
     if not 'msvs_error_on_missing_sources' in gyp_generator_flags:
       os.environ[ENVVAR_GYP_GENERATOR_FLAGS] = (
           gyp_generator_flags + ' msvs_error_on_missing_sources=1')
@@ skipping 15 matching lines @@
     if res:
       sys.exit(res)
 
   # This code is copied from Chrome's build/gyp_chromium. It's not clear why
   # the *_runtime variables are reversed.
   if vs2013_runtime_dll_dirs:
     x64_runtime, x86_runtime = vs2013_runtime_dll_dirs
     vs_toolchain.CopyVsRuntimeDlls(
         os.path.join(os.getenv('CHROME_PATH'), get_output_dir()),
         (x86_runtime, x64_runtime))