Move default argument to declaration.

Fixes clang warning "addition of default argument on redeclaration makes
this constructor a default constructor".
Revert "Remove os::thread_specific_ptr"

This reverts commit 67ec200baea241d55bea33c43fd5194d90d9e401.
os: Simplify os::thread_specific_ptr.

Instead of mimicking boost::thread_specific_ptr, do something that allows
to mimick more closely compiler TLS support.
os: Fallback to OS TLS when compiler does not support it (issue #120).
glretrace: Use OS_THREAD_SPECIFIC_PTR on glws_cocoa.mm
os: Bring back the compiler TLS check.

Seems better than fiddling with pre-defined macros.