Merge #18709: doc: note why we can't use thread_local with glibc back compat

b155fcda51 doc: fix typo in configure.ac (fanquake)
20a30922fb doc: note why we can't use thread_local with glibc back compat (fanquake)

Pull request description:

  Given that we went through a [gitian build](https://github.com/bitcoin/bitcoin/pull/18681) to remember why this is the case, we might as well make a note of it in configure.ac.

  [From #18681](https://github.com/bitcoin/bitcoin/pull/18681#issuecomment-615526634):

  Looking at the Linux build log, this has failed with:
  ```bash
  Checking glibc back compat...
  bitcoind: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  bitcoind: failed IMPORTED_SYMBOLS
  bitcoin-cli: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  bitcoin-cli: failed IMPORTED_SYMBOLS
  bitcoin-tx: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  bitcoin-tx: failed IMPORTED_SYMBOLS
  bitcoin-wallet: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  bitcoin-wallet: failed IMPORTED_SYMBOLS
  test/test_bitcoin: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  test/test_bitcoin: failed IMPORTED_SYMBOLS
  bench/bench_bitcoin: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  bench/bench_bitcoin: failed IMPORTED_SYMBOLS
  qt/bitcoin-qt: symbol __cxa_thread_atexit_impl from unsupported version GLIBC_2.18
  ```

  `__cxa_thread_atexit_impl` is used for [thread_local variable destruction](https://sourceware.org/glibc/wiki/Destructor%20support%20for%20thread_local%20variables):

  > To implement this support, glibc defines __cxa_thread_atexit_impl exclusively for use by libstdc++ (which has the __cxa_thread_atexit to wrap around it), that registers destructors for thread_local variables in a list. Upon thread or process exit, the destructors are called in reverse order in which they were added.

  As suggested, this only became available in glibc 2.18. From the [2.18 release notes](https://sourceware.org/legacy-ml/libc-alpha/2013-08/msg00160.html):

  > * Add support for calling C++11 thread_local object destructors on thread
    and program exit.  This needs compiler support for offloading C++11
    destructor calls to glibc.

ACKs for top commit:
  hebasto:
    ACK b155fcda51

Tree-SHA512: 5b9567e4a70598a4b0b91956f44ae0d93091db17c84cbf9817dac6cfa992c97d3438a8b1bb66644c74891f2149e44984daed445d22de93ca8858c5b0eabefb40

configure.ac

@@ -256,7 +256,7 @@ AC_ARG_ENABLE([gprof],
     [enable_gprof=$enableval],
     [enable_gprof=no])
 
-dnl Pass compiler & liner flags that make builds deterministic
+dnl Pass compiler & linker flags that make builds deterministic
 AC_ARG_ENABLE([determinism],
     [AS_HELP_STRING([--enable-determinism],
                     [Enable compilation flags that make builds deterministic (default is no)])],
@@ -861,6 +861,9 @@ 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"