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

b155fcda5186c59fc4fb2a9eaaf791d132e0ab30 doc: fix typo in configure.ac (fanquake)
20a30922fbf6ba14e250ca649239af115dbbe7b0 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 b155fcda5186c59fc4fb2a9eaaf791d132e0ab30

Tree-SHA512: 5b9567e4a70598a4b0b91956f44ae0d93091db17c84cbf9817dac6cfa992c97d3438a8b1bb66644c74891f2149e44984daed445d22de93ca8858c5b0eabefb40

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