Avoid thread_local on current or older MinGW.

This is for jtv#453.  Turns out MinGW gcc's current `thread_local`
implementation breaks _at runtime._  So a simple compilation check won't
detect the problem, but it's a serious bug, reportedly fixed in gcc 11.

I'll still need to re-generate the configure script etc.  But I'll do
that in a separate commit.

config-tests/thread_local.cxx

@@ -4,6 +4,9 @@
 
 int main(int argc, char **)
 {
+#if defined(__MINGW32__) && defined(__GNUC__) && __GNUC__ < 11
+#error "This MinGW gcc's thread_local breaks at run time, sorry."
+#endif
   thread_local std::stringstream s;
   s << argc;
   std::cout << s.str();