Move OS/compiler/feature detection to public v8config.h header.

From now on the v8config.h header should be the one and
only file where we do (freaky) checks to detect OS, C++
compiler or certain compiler features. Since we need that
both internally and for the public API, the new v8config.h
is the proper place to add (everything is prefixed with V8_
so we are safe).

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23248006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16281 ce2b1a6d-e550-0410-aec6-3dcde31c8c00