build: fix build for SmartOS

This change in V8: https://code.google.com/p/v8/source/detail?r=22210
has introduced a method named OS::GetCurrentThreadId which fails to
compile on OSes where a "gettid" syscall does not exist.

This build issue has been fixed upstream by several changes:
- https://code.google.com/p/v8/source/detail?r=23459.
- https://codereview.chromium.org/649553002
- https://codereview.chromium.org/642223003

Another minor fix to the upstream changes was also necessary.
See https://code.google.com/p/v8/issues/detail?id=3620 for
more information.

The other build issue was due to the fact that alloca.h is not included
by other system includes on SmartOS, which is assumed by V8.

Built and tested on Linux, MacOS X, Windows and SmartOS.