This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

Modeled after std::filesystem::operator/() from C++17.

This is just the API design; there is no implementation yet, and no
caller.

This method was added in Boost 1.58.

Not updating m4/ax_boost_base.m4 because the new version breaks
cross-compilation by adding -I/usr/include to BOOST_CPPFLAGS.

Closes #296