Add support for replacing Boost.Fileystem with C++17 <filesystem> when available

[cmazakas]

Boost.Process is a wonderful library but existed for longer than C++17 compilers have and as such, its API is polluted with boost::filesystem::path. This causes an awkward disconnect when being used in C++17 projects.

This issue aims to start a discussion addressing this issue and the best path forward for giving C++17 users a way of using std::filesytem::path.

[jbgomond]

+1 for with that one ! It seems that Boost DLL is using the flag BOOST_DLL_USE_STD_FS for backword compatibility with C++11.

[laanwj]

+1 to this, we ran into a similar issue

[ingo-loehken]

+1 for this. should be removed or be configurable like for boost.dll.