CMakeLists.txt adding std++fs linking

[jgalan]

I found out that I need to add this fix in order to be able to compile at sultan2, strange ... but

[lobis]

I found out that I need to add this fix in order to be able to compile at sultan2, strange ... but

This is because we are using gcc version 8, which I think its fairly old? (but still recieving updates?), I am not sure how it works, but for example in my dev container using Ubuntu 22.04 the default is version 11.

[jgalan]

It is always good to keep compatibility with as older compiler as possible since this will allow REST to be installed at any system.