message: [Build] Add flag for enabling thread_local
  When aiming for glibc compatibility, don't use thread_local.
  Ensures we're meeting the backwards compatibility target with gitian.
  Add a flag --enable-threadlocal, which, when specified, will
  enable/disable thread_local regardless of the value of glibc_compat.
  MingW, Darwin, and FreeBSD have a buggy thread_local, don't use it.
diff: @@ -181,6 +181,12 @@ 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([--enable-asm],
  [Enable assembly routines (default is yes)])],
  @@ -800,6 +806,7 @@ 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])
  @@ -813,14 +820,35 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
  }
  ])],
  [
  + case $host in
  + *mingw*)
  + dnl mingw32's implementation of thread_local has also been shown to behave
  + dnl erroneously under concurrent usage; see:
  + dnl https://gist.github.com/jamesob/fe9a872051a88b2025b1aa37bfa98605
  + AC_MSG_RESULT(no)
  + ;;
  + *darwin*)
  + dnl TODO enable thread_local on later versions of Darwin where it is
  + dnl supported (per https://stackoverflow.com/a/29929949)
  + AC_MSG_RESULT(no)
  + ;;
  + *freebsd*)
  + dnl FreeBSD's implementation of thread_local is also buggy (per
  + dnl https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)
  + AC_MSG_RESULT(no)
  + ;;
  + *)
  AC_DEFINE(HAVE_THREAD_LOCAL,1,[Define if thread_local is supported.])
  AC_MSG_RESULT(yes)
  + ;;
  + esac
  ],
  [
  AC_MSG_RESULT(no)
  ]
  )
  LDFLAGS="$TEMP_LDFLAGS"
  +fi
  # Check for different ways of gathering OS randomness
  AC_MSG_CHECKING(for Linux getrandom syscall)
  