Building on Ubuntu 18.04 no longer works as documented
```
CMake Error at lib/settings/CMakeLists.txt:132 (message):
GCC version must be at least 8.0 if using std filesystem
```
I verified the build instructions on Ubuntu 20.04 and they work as advertised.
For Ubuntu 18.04, you'd have the option of
1) Manually installing a later version of GCC
2) Using clang
3) passing the `-DPAJLADA_SETTINGS_USE_BOOST_FILESYSTEM=1` flag to cmake
* Updated required packages for arch
Also added thread number to make, so it won't build with just 1 thread.
* Update BUILDING_ON_LINUX.md
Co-authored-by: fourtf <tf.four@gmail.com>
* Clean-up build instructions for Fedora
- separate development vs. optional dependencies
- change use of `yum` to `dnf`
- fix typo from `streamer-plugins-good` to `gstreamer-plugins-good`
- mention info regarding `gstreamer-plugins-good` being retired in f31