Linux: make ninja respect environment:

- CFLAGS
- CXXFLAGS
- LDFLAGS

BUG=251122, gyp:360
R=evan@chromium.org, scottmg@chromium.org

Review URL: https://codereview.chromium.org/17359002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1691 78cadc50-ecff-11dd-a971-7dbc132099af