SkPreConfig.h: Allow defined(__unix__) to imply SK_BUILD_FOR_UNIX

This is more general than checking for __linux, __FreeBSD__, etc.  (In
principle we could remove some of the existing checks such as
__FreeBSD__, but I have not tried that so far.)

In particular, it allows Skia to build with the NaCl or PNaCl
toolchains, which is something we would like for Mojo.

BUG=https://github.com/domokit/mojo/issues/431
TEST=none

Review URL: https://codereview.chromium.org/1523733003
1 file changed