These updates fix building the project on my up to date Linux box due to
missing cstdint headers on the older Draco versions it was pinned to,
and bumps Conan 1.x to the latest semver-compatible release, which also
introduces support for newer GCC compilers.
While at it, I've also bumped the GitHub Actions checkout version tag to
v4, from v3, and the runner image versions. The Ubuntu runner version
was purposefully not updated to avoid the risk of making binaries less
portable than before due to a bump in the `glibc` version that the
compiler will link to.
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.
