bake command line option to set defines

[aschaal]

Instead of having multiple build configurations:

ExecutableConfig configIPV4 {
  DefaultToolchain GCC {
    Compiler CPP {
      Define "USE_IPV4"
    }
  }
}

ExecutableConfig configIPV6 {
  DefaultToolchain GCC {
    Compiler CPP {
      Define "USE_IPV6"
    }
  }
}

It would be more convenient to call it like this:

ExecutableConfig configIPVx {
  DefaultToolchain GCC
}

bake configIPVx -D USE_IPV4
bake configIPVx -D USE_IPV6

[aschaal]

Note to myself: if "-D" changes, the cache has to be invalided.

[aschaal]

Done in 2.47.0