Don't require -DSK_USE_POSIX_THREADS.

Don't require -DSK_USE_POSIX_THREADS.

To compile SkCondVar, we already require either pthreads or Windows.  This
simplifies that code to not need SK_USE_POSIX_THREADS to be explicitly defined.
We'll just look to see if we're targeting Windows, and if not, assume pthreads.

Both before and after this CL, that code will fail to compile if we're not on
Windows and don't have pthreads.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/4daa6f613cb7d8a3c07369af3e919ea65405ec55

[mtklein, 2015-01-22 03:22:19]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-01-22 03:22:31]

mtklein@google.com changed reviewers:
+ bungeman@google.com, reed@google.com

[commit-bot: I haz the power, 2015-01-22 03:23:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869443003/1

[commit-bot: I haz the power, 2015-01-22 03:23:17]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-01-22 09:23 UTC

[bungeman-skia, 2015-01-22 03:49:24]

I think SK_USE_POSIX_THREADS used to be used to select things like mutex and
thread implementations, but then that moved to the build system and is now just
platform dependent. Making these just platform dependent also is at least
consistent.

lgtm

[commit-bot: I haz the power, 2015-01-22 03:51:32]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/4daa6f613cb7d8a3c07369af3e919ea65405ec55