[build] Warn that only libconsensus can be built without boost
Summary:
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".
Backport of core PR11936
https://github.com/bitcoin/bitcoin/pull/11936/files
Test Plan:
./autogen.sh mkdir build aa cd build ../configure --with-boost=no
Check the output:
configure: error: only libbitcoinconsensus can be built without boost
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3547