It doesn't cover all cases, but we can work on those as
we go along. For now, gcc, MSVC++, Intel C/C++, IBM XL C/C++,
Sun Studio C/C++ and Borland C++ Builder are the supported
compilers (although we boldly assume that they all are of
a recent enough version to support thread-local storage).

This is intended to be used in upcoming patches that implement
graceful (but TLS-dependant) error-handling in the library.

As an added bonus, we also bring the online_cpus() function
from git.git to detect the number of usable cpu's.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>