[Bug libgcc/89714] allow for overriding the thread-detection mechanism

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89714

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That wouldn't help when people link directly with -lpthread rather than
-pthread. _REENTRANT is not required for pthreads, so I don't think we should
depend on it.