std::filesystem not found in default build

[CharlesGueunet]

Building after a default configuration leads to an error because std::filesystem is not found.
Changing BLT_CXX_STD from c++14 to c++17 solve the issue.
The std::filesystem include should be protected or the support of c++14 dropped.

[barracuda156]

Well, POSIX back-end works fine (which is picked with C++14), but perhaps the default should rather be C++17.