Add recommended build flags to the Ninja file generator

[rdw-software]

I didn't spend much time researching which ones should be set. Pretty sure the defaults aren't too great, since there's a lot of legacy cruft that prevents toolchain vendors from enabling things that really should be enabled nowadays...
Will have to see what features seem useful and update the build configuration accordingly.

See https://developers.redhat.com/blog/2018/03/21/compiler-and-linker-flags-gcc for an overview (?)

[rdw-software]

Should also remove the debug symbols before a proper release is published...