Allow YCM to use a custom output_dir from GYP_GENERATOR_FLAGS env variable.
Detects and uses a custom build directory specified by GYP_GENERATOR_FLAGS as
per the instructions on the chromium wiki:
https://code.google.com/p/chromium/wiki/CommonBuildTasks#Change_Build_Output_Directory
BUG=None
TEST=YCM provides completions for custom output_dir.
NOTRY=true (no code changes)
Review URL: https://codereview.chromium.org/
248773004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@265762 0039d316-1c4b-4281-b951-d872f2087c98