Fix Makefile for Ubuntu 18.04

[valeriob01]

The <filesystem> lib not found. Fix:

CXX=g++-8

[valeriob01]

Also the library stdc++fs is not available anymore in Ubuntu.

[preda]

Why Ubuntu 18.04? it's a bit old, with 19.10 coming out soon.

[valeriob01]

Why Ubuntu 18.04? it's a bit old, with 19.10 coming out soon.

Effectively this comment was on July 29...

[valeriob01]

Why Ubuntu 18.04? it's a bit old, with 19.10 coming out soon.

Yes it may sound anachronistic, but amdgpu-pro 19.30 only installs on Ubuntu 18.04 at the moment.

[preda]

Is there anything left to do here? I don't want to mandate a specific g++ version in the makefile, which may be different from what the user has installed. If the default g++ is not good, the user can update the makefile as needed.

[selroc]

Is there anything left to do here? I don't want to mandate a specific g++ version in the makefile, which may be different from what the user has installed. If the default g++ is not good, the user can update the makefile as needed.

I think this issue can be closed, that was a special case.