avoid mis-mangled name references on threaded C++ builds

avoid mis-mangled name references on threaded C++ builds

g++ on recent darwin appears to be clang++ in disguise, and so
supports the TSA decorations, but the wrappers we define don't have
explicit extern "C" linkage (which perlapi.h does have).

Under C++ Time::HiRes wraps its includes in extern "C" { }, and
so references unmangled names for the perl_tsa_mutex_*() wrappers,
but since util.c never sees an extern "C" for those functions it
produces mangled names, causing undefined references in Time::HiRes.

Detected by a smoke run againt a smoke-me branch:

http://www.nntp.perl.org/group/perl.daily-build.reports/2016/03/msg190183.html

Longer term the TSA decorations should probably be probed for in Configure
and the wrapper declarations moved to embed.fnc