ninja windows: Fully qualify path to cl.exe

Part of upgrading to ninja trunk with more efficient dependency
tracking.

Does path search for correct cl.exe at generation time rather than
relying on the path being set properly.

Supports CC_wrapper/CXX_wrapper for goma/distcc. *_wrapper variables
are now supported from the environment on all platforms.

R=thakis@chromium.org
BUG=chromium:232421

Review URL: https://codereview.chromium.org/13950005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1612 78cadc50-ecff-11dd-a971-7dbc132099af
Dropping the scons generator.

The scons generator has not been used for some time and is known to no longer
be at parity with the other generators.
Dropping it to reduce the ongoing maintenance burden.

BUG=None
TEST=None
R=mark@chromium.org

Review URL: https://codereview.chromium.org/13869005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1613 78cadc50-ecff-11dd-a971-7dbc132099af