Fix bug in the way pthead_mutex_t was being destroyed in win32.
Win32 critical section objects (CRITICAL_SECTION) are not kernel objects.
Only kernel objects are destroyed by using CloseHandle. Critical sections
are supposed to be deleted with the DeleteCriticalSection API
(http://msdn.microsoft.com/en-us/library/ms682552(VS.85).aspx).
Add proper threading support to libgit2
We now depend on libpthread on all Unix platforms (should be installed
by default) and use a simple wrapper for Windows threads under Win32.
Signed-off-by: Vicent Marti <tanoku@gmail.com>