GCC 8 Required Link to Filesystem

[connorfuhrman]

I'm compiling ProtoGen on a Raspberry Pi with GCC version Raspbian 8.3.0-6+rpi1 and needed to add unix:LIBS += -lstdc++fs to the ProtoGen.pro file to overcome linker errors to things in std::filesystem. One can get around this by using a newer compiler (apparently GCC 9+ doesn't need -lstdc++fs per https://stackoverflow.com/questions/53852684/stdfilesystem-link-error-on-ubuntu-18-10) but if using GCC8 add the extra linker flag if you're running into linking errors.

Perhaps this could be added to the QT project file or just a disclaimer on the README for older compilers?