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:

Review URL: https://codereview.chromium.org/650553002
1 file changed