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 another change:
https://code.google.com/p/v8/source/detail?r=23459. This commit
integrates this fix. It's still not clear if this is good enough for the
long term, 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.