On a system with GCC 5.* Conan will conservatively choose 'libstdc++'
over 'libstdc++11' for compiler.libcxx, and then proceed to download
libraries compiled with the older ABI.
Meanwhile, though, our own CMake setup dictates the use of the modern
ABI, and the result is an application binary with ABI mismatches that
yield SIGSEGVs almost immediately.
Here, we guard against erronous invocations, and gently push the user
towards sending in the right explicit override for their system.
With this, we are able to get rid of all the increasingly broken file
system utility code, and trust boost::filesystem to handle all the
cross-platform complexity.
The first version of this PR centred around C++17 & std::filesystem,
but support remains too elusive; it seems works out of the box in
Visual Studio (especially 2019), but is entirely missing from the Mac
dclang, and even with GCC 8.0 it requires an explicit '-l c++fs'.
Luckily the std:: version is almost exactly the boost:: version (not
surprising) so when the world's caught up, we can ditch Boost and go
all stdlib.
Setting up Conan requires a bit of work; we'll want to document the
details in the README.
