Add option for link concurrency.

Skia's buildbots, particularly Windows builds with LTO, are held back by the
link pool.  We'd like to play around with this number a bit.  Any problem
checking an environment variable first before doing the RAM-based calculation?

Testing: ran with and without GYP_LINK_CONCURRENCY=90, looked at

Patch from

Review URL:

git-svn-id: 78cadc50-ecff-11dd-a971-7dbc132099af
1 file changed