configure: Add flag for enabling thread_local.

- When aiming for glibc compatibility, don't use thread_local.
- Add a flag --enable-threadlocal, which, when specified, will
  enable/disable thread_local regardless of the value of glibc_compat.
- FreeBSD has a buggy thread_local, don't use it.

configure.ac

@@ -194,6 +194,12 @@ AC_ARG_ENABLE([glibc-back-compat],
   [use_glibc_compat=$enableval],
   [use_glibc_compat=no])
 
+AC_ARG_ENABLE([threadlocal],
+  [AS_HELP_STRING([--enable-threadlocal],
+  [enable features that depend on the c++ thread_local keyword (currently just thread names in debug logs). (default is to enabled if there is platform support and glibc-back-compat is not enabled)])],
+  [use_thread_local=$enableval],
+  [use_thread_local=auto])
+
 AC_ARG_ENABLE([asm],
   [AS_HELP_STRING([--disable-asm],
   [disable assembly routines (enabled by default)])],
@@ -827,10 +833,11 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
   ]
 )
 
-TEMP_LDFLAGS="$LDFLAGS"
-LDFLAGS="$TEMP_LDFLAGS $PTHREAD_CFLAGS"
-AC_MSG_CHECKING([for thread_local support])
-AC_LINK_IFELSE([AC_LANG_SOURCE([
+if test "x$use_thread_local" = xyes || { test "x$use_thread_local" = xauto && test "x$use_glibc_compat" = xno; }; then
+  TEMP_LDFLAGS="$LDFLAGS"
+  LDFLAGS="$TEMP_LDFLAGS $PTHREAD_CFLAGS"
+  AC_MSG_CHECKING([for thread_local support])
+  AC_LINK_IFELSE([AC_LANG_SOURCE([
     #include <thread>
     static thread_local int foo = 0;
     static void run_thread() { foo++;}
@@ -852,6 +859,11 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
           # supported (per https://stackoverflow.com/a/29929949)
           AC_MSG_RESULT(no)
           ;;
+        *freebsd*)
+          # FreeBSD's implementation of thread_local is also buggy (per
+          # https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)
+          AC_MSG_RESULT(no)
+          ;;
         *)
           AC_DEFINE(HAVE_THREAD_LOCAL,1,[Define if thread_local is supported.])
           AC_MSG_RESULT(yes)
@@ -861,8 +873,9 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
     [
       AC_MSG_RESULT(no)
     ]
-)
-LDFLAGS="$TEMP_LDFLAGS"
+  )
+  LDFLAGS="$TEMP_LDFLAGS"
+fi
 
 # Check for different ways of gathering OS randomness
 AC_MSG_CHECKING(for Linux getrandom syscall)