Fix libegt/egt-launcher with clang/libc++

[kraj]

@noglitch support for std::filesystem is still needed to get the latest libegt working. using c++17 is fixed in this version which is great but code for experimental::filesystem also needs to change

[gehariprasath]

Hello Khem

Thanks for your patch. Pls be assured that this is being looked at with our internal EGT team and we will respond to this at the earliest.

[ldesroches]

Thanks, for these patches. It's fine, PR can be merged. We'll integrate the in EGT for the next release.

[noglitch]

Merged. Thanks @kraj !