Merge bitcoin/bitcoin#24397: build: Fix Boost.Process check for Boost 1.73 and older

774323e378acea998069980edf50e1a26f4b6b9e ci: Force `--enable-external-signer` to prevent future regressions (Hennadii Stepanov)
69978858a4d8a9c34070f96133b8f95737a6505b build: Fix Boost.Process check for Boost 1.73 and older (Hennadii Stepanov)
2199ef79cba865af7949c3a55938ad54519ca5b8 build: Fix a non-portable use of `test` (Hennadii Stepanov)
d436c488d4216298016b09f8f97e10eada489bda build, refactor: Replace tabs with spaces (Hennadii Stepanov)

Pull request description:

  On master (5f44c5c428b696af4214b2519cb2bbeb0e4a1027) Boost.Process check false fails without the `-lpthread` flag.

  ```
  $ grep -C 2 pthread_detach config.log
  /usr/bin/ld: /tmp/cczCQfQv.o: in function `boost::asio::detail::posix_global_impl<boost::asio::system_context>::~posix_global_impl()':
  conftest.cpp:(.text._ZN5boost4asio6detail17posix_global_implINS0_14system_contextEED2Ev[_ZN5boost4asio6detail17posix_global_implINS0_14system_contextEED5Ev]+0xa3): undefined reference to `pthread_join'
  /usr/bin/ld: conftest.cpp:(.text._ZN5boost4asio6detail17posix_global_implINS0_14system_contextEED2Ev[_ZN5boost4asio6detail17posix_global_implINS0_14system_contextEED5Ev]+0xc4): undefined reference to `pthread_detach'
  collect2: error: ld returned 1 exit status
  configure:26674: $? = 1
  ```

  Not required for Boost 1.74+.

ACKs for top commit:
  laanwj:
    Code review ACK 774323e378acea998069980edf50e1a26f4b6b9e, is a bugfix/workaround, seems fine to merge last minute for 23.0.

Tree-SHA512: 2a9d4b67fd8910e107af972d8c223286b7c933bc310616f86c8b6d8c903438916980fc76bd7e37f2698f6ce5361dc706cbf2933d1ac2c081bcabe1b83ca7d6b6

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