Merge #11936: [build] Warn that only libconsensus can be built without Boost

Merge #11936: [build] Warn that only libconsensus can be built without Boost

3eb4d45 [build] Warn that only libconsensus can be built without boost (Varunram)

Pull request description:

  This replaces the "configure: error: Could not find a version of the boost_system library!" message you receive when trying to build without Boost, with "only libbitcoinconsensus can be built without boost".

  `./configure --with-utils=no --disable-bench --disable-gui-tests --disable-tests --with-daemon=no --without-gui --disable-wallet --with-boost=no` builds libconsensus.

  `./configure --with-boost=no` should always fail with:
  ```
  checking whether to build Bitcoin Core GUI... yes (Qt5)
  configure: error: only libbitcoinconsensus can be built without boost
  ```

  For anyone wondering why the check comes after the AX_BOOST_BASE check, see this [comment](https://github.com/bitcoin/bitcoin/pull/11806#discussion_r155359394). "the AX_BOOST_BASE macro that does the --with-boost handling (along with the actual checks), and sets "want_boost". "

  Fixes #10826, replaces #11806.

  @theuni if you re-ACK we can get this merged.

Tree-SHA512: f0b8f483586465187ca6689e731b24ff77da41a06fb5c9d6390c82990719911dd54ebcccaf6d4fcea2be92986cc7fa88ed979e6cb9d77917920181e5e5188067