Link stdc++fs for older gcc versions

[soheilshahrouz]

Fixes this issue.

According to filesystem library documentation, gcc versions older than 9.1 and clang prior to 9.0 need filesystem library to be explicitly linked. This PR adds the compiler option when it is necessary.

When trying to reproduce the compilation failure, I realized gcc-8.4 does not need the additional compiler option. However, to be consistent with the official documentation, I add the compiler flag for versions older than 9.1.

How Has This Been Tested?

Current Github Ubuntu images do not have gcc-8 installed by default. When I manually installed gcc-8.4, VTR was built without any problem. It seems that it is only gcc-8.3 and older version sthat need the link flag. I reproduced the compilation error on my local machine by building gcc-8.3 from source. Adding the compiler flag fixed the error. CI tests show that the flag is not applied for newer gcc versions.

Types of changes

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[vaughnbetz]

Looks good, thanks!