Merge #21182: build: remove mostly pointless BOOST_PROCESS macro

7bf04e358a6550ac9851f1b2d87795927fc5ff4b build: remove mostly pointless BOOST_PROCESS macro (fanquake)

Pull request description:

  Performing a series of link checks for a Boost component that is
  header-only doesn't make much sense, and currently means we just have
  another confusing Boost macro in our tree. I'm not sure why this was
  originally done this way; maybe Sjors or luke-jr can elaborate (#15382 (929cda5470f98d1ef85c05b1cad4e2fb9227e3b0))?

  The macro also has the side-effect of producing confusing error
  messages. i.e in #20744, the CI is currently failing with:
  ```bash
  checking for boostlib >= 1.58.0 (105800) lib path in "/tmp/cirrus-ci-build/depends/x86_64-pc-linux-gnu/lib"... yes
  checking for boostlib >= 1.58.0 (105800)... yes
  checking whether the Boost::Process library is available... yes
  configure: error: Could not find a version of the Boost::Process library!
  ```

  This isn't useful, given there is no such thing as a `Boost::Process` library.

  This PR just removes the macro entirely, but maintains a `--with-boost-process`
  (defaulting to off), flag to configure. Hopefully this will also be
  removed, in favour of `--enable/disable-external-signer` if/when #16546
  is merged.

ACKs for top commit:
  laanwj:
    ACK 7bf04e358a6550ac9851f1b2d87795927fc5ff4b

Tree-SHA512: b270a0250f32df2078f986c165b8977967d8c06df80bf2773f3442f74b395a3bfa6544af1024d9b6524d90d47a0f6304194b3aced0e2ecb88e75916da945ccb6

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