message: doc: note why we can't use thread_local with glibc back compat
diff: @@ -903,6 +903,9 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
  ]
  )
  +dnl thread_local is currently disabled when building with glibc back compat.
  +dnl Our minimum supported glibc is 2.17, however support for thread_local
  +dnl did not arrive in glibc until 2.18.
  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"
  