Makefile.m32: reintroduce CROSSPREFIX and -W -Wall [ci skip]

Makefile.m32: reintroduce CROSSPREFIX and -W -Wall [ci skip]

- Reintroduce `CROSSPREFIX`:

  If set, we add it to the `CC` and `AR` values, and to the _default_
  value of `RC`, which is `windres`. This allows to control each of
  these individidually, while also allowing to simplify configuration
  via `CROSSPREFIX`.

  This variable worked differently earlier. Hopefully this new solution
  hits a better compromise in usefulness/complexity/flexibility.

  Follow-up to: aa970c4c08775afcd0c2853be89b0a6f02582d50

- Enable warnings again:

  This time with an option to override it via `CFLAGS`. Warnings are
  also enabled by default in CMake, `makefile.dj` and `makefile.amiga`
  builds (not in autotools though).

  Follow-up to 10fbd8b4e3f83b967fd9ad9a41ab484c0e7e7ca3

Closes #9784