Using Boost.Test header-only from CMake

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-06-07 01:11:49

Some libraries (I encountered this when porting the tests of Signals2)
use Boost.Test in header-only mode, by including <boost/test/included/unit_test.hpp>
without linking to anything. This worked in the "legacy" structure where
all headers are always available, but doesn't work under CMake where
you have to link to header-only libraries in order to get the proper include
path.

Should we make an attempt to support this use by declaring a special
header-only target in Boost.Test (e.g. Boost::included_unit_test), or is
that a waste of time? One can as well link to the proper
Boost::unit_test_framework (and then even use it header-only, although
there's not much point in doing so.)

I'm inclined towards "waste of time".