The Mach-O format does not permit gcc to implement the __thread
TLS specification, so we must instead emulate it using a single
int cell allocated from memory and stored inside of the thread
specific data associated with the current pthread.

What makes this tricky is git_errno must be a valid lvalue, so
we really need to return a pointer to the caller and deference it
as part of the git_errno macro.

The GCC-specific __attribute__((constructor)) extension is used
to ensure the pthread_key_t is allocated before any Git functions
are executed in the library, as this is necessary to access our
thread specific storage.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>