[GTK] Use win32 native threading

[GTK] Use win32 native threading
https://bugs.webkit.org/show_bug.cgi?id=77676

Patch by Kalev Lember <kalevlember@gmail.com> on 2012-02-02
Reviewed by Martin Robinson.

r97269 switched from glib threading to pthreads, breaking win32 GTK+.
This is a follow up, removing some leftovers in ThreadSpecific.h and
switching win32 to use the native threading in ThreadingWin.cpp.

* GNUmakefile.list.am: Compile in win32 native threading support
* wtf/ThreadSpecific.h: Remove GTK+-specific definitions
(ThreadSpecific):
(WTF::::destroy):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@106598 268f45cc-cd09-0410-ab3c-d52691b4dbfc