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. "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.