Freeciv

We currently set `-Wall` as a compiler option along with the other `-W` options, then apply that to the C and C++ compiler.

Meson warns us about this each time.

Compiler for C supports arguments -Wall: YES 
../meson.build:53: WARNING: Consider using the built-in warning_level option instead of using "-Wall".
Compiler for C++ supports arguments -Wall: YES 
../meson.build:58: WARNING: Consider using the built-in warning_level option instead of using "-Wall".

We should make the suggested change as it:

1. enables users or downstreams to configure warnings as they need / desire
2. reduces configure noise
3. is automatically applied to supported compilers
4. is the 'meson way'