Merge #16059: configure: Fix thread_local detection

480e3415d7 configure: Add flag for enabling thread_local. (Carl Dong)

Pull request description:

  - When aiming for glibc compatibility, don't use thread_local. Fixes #15958.
  - FreeBSD has a buggy thread_local, don't use it. Fixes #16055.

  I've done a Gitian build on my local machine and the symbol tests seem to pass.

ACKs for commit 480e34:
  MarcoFalke:
    utACK 480e3415d738a4e0e2ad9774c43f29937178ecae
  fanquake:
    tACK 480e341

Tree-SHA512: 334f21f7cf271c261b115a6410afd4ed4db3e84ad79b98c6c684c1dfa42b081f16d58e77695929e27b0fa173a894b959a327fe82821a3f3ed708b305a906ddd3

configure.ac

@@ -195,6 +195,12 @@ 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)])],
@@ -876,6 +882,7 @@ 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 $PTHREAD_CFLAGS"
   AC_MSG_CHECKING([for thread_local support])
@@ -901,6 +908,11 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
           # supported (per https://stackoverflow.com/a/29929949)
           AC_MSG_RESULT(no)
           ;;
+        *freebsd*)
+          # FreeBSD's implementation of thread_local is also buggy (per
+          # https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)
+          AC_MSG_RESULT(no)
+          ;;
         *)
           AC_DEFINE(HAVE_THREAD_LOCAL,1,[Define if thread_local is supported.])
           AC_MSG_RESULT(yes)
@@ -912,6 +924,7 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
     ]
   )
   LDFLAGS="$TEMP_LDFLAGS"
+fi
 
 dnl check for gmtime_r(), fallback to gmtime_s() if that is unavailable
 dnl fail if neither are available.