An unhappy conjunction of GCC bug #56480 [1] and Boost having
different namespaces for the print_log_value make it quite
ugly to support this method.
For the limited purposes of the unit tests, a free function in
the unit_test_utils header (in the absence of any implementation
in the main libraries) will do, even if it is a little intrusive.
From Boost 1.64 onwards, the customisation point boost_test_print_type
is avaiable, and anyt print functions should be transitioned over to
that method when the minimum Boost version is 1.64 or higher.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480
97a37a74f8