Allow GCC to build Skia on Mac.

Allow GCC to build Skia on Mac.

GCC doesn't understand -fasm-blocks or -mpascal-strings, but we don't care
about them.

While looking around in Gyp, I noticed a better way to disable warnings about
offsetof so that it doesn't tell us "disabling this warning makes no sense in C"
for every C source file we compile.  

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/335a0ae19c3c26a934b2a25c0b8016a453c4b013

[mtklein, 2014-10-10 23:09:45]

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

[mtklein, 2014-10-10 23:09:49]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-10 23:10:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/650553002/1

[commit-bot: I haz the power, 2014-10-10 23:10:40]

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 2014-10-11 05:10 UTC

[commit-bot: I haz the power, 2014-10-11 05:10:47]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-11 05:10:48]

No LGTM from a valid reviewer yet. Please ask for an LGTM from a
full Skia committer from
https://skia.googlesource.com/skia/+/master/CQ_COMMITTERS

[reed1, 2014-10-13 12:25:21]

lgtm

[mtklein, 2014-10-13 13:14:48]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-13 13:15:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/650553002/1

[mtklein, 2014-10-13 19:29:45]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2014-10-13 19:29:48]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-13 19:32:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/650553002/1

[commit-bot: I haz the power, 2014-10-13 19:32:46]

Committed patchset #1 (id:1) as 335a0ae19c3c26a934b2a25c0b8016a453c4b013

[commit-bot: I haz the power, 2014-10-13 19:32:49]

Failed to apply the patch.