build: Enable thread_local for MinGW-w64 builds

The assumption in the commit 188ca75
about the broken `thread_local` implementation in GCC was misguided
because the initial failure was not due to GCC, but a bug in the Wine
runtime, as evidenced, for example, here:
 - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=917307
 - https://bugs.freedesktop.org/show_bug.cgi?id=108662

Consequently, it is safe to re-enable `thread_local` support for
MinGW-w64 builds.

configure.ac

@@ -1042,13 +1042,7 @@ if test "$use_thread_local" = "yes" || test "$use_thread_local" = "auto"; then
     }
     ])],
     [
-     case $host in
-       *mingw*)
-          dnl mingw32's implementation of thread_local has also been shown to behave
-          dnl erroneously under concurrent usage; see:
-          dnl https://gist.github.com/jamesob/fe9a872051a88b2025b1aa37bfa98605
-          AC_MSG_RESULT([no])
-          ;;
+      case $host in
         *freebsd*)
           dnl FreeBSD's implementation of thread_local is also buggy (per
           dnl https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)