Override the user's language settings during tests

If a user's language is set to anything other than English, the output
of various tools may vary from what is expected and cause tests to
fail. This fixes several test failures for non-English users.

To reproduce the issue, run: LC_ALL=de_DE.UTF-8 ./gyptest.py -a

Patch by Joey Parrish <joeyparrish@google.com>

Review URL: https://codereview.chromium.org/324893007/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1932 78cadc50-ecff-11dd-a971-7dbc132099af
ninja win: Remove obsolete comment, and make sure cc/cxx are always overridden

Builds on discussion in https://codereview.chromium.org/321813002/.

Make sure that cc_host and cxx_host are also set to a valid compiler,
which fixes the cflags test.

R=yukawa@chromium.org
BUG=chromium:233985

Review URL: https://codereview.chromium.org/324383004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1933 78cadc50-ecff-11dd-a971-7dbc132099af