STYLE: cleanup of compiler settings

- relocate WM_COMPILER_ARCH, WM_COMPILER_LIB_ARCH from
  etc/{bashrc,cshrc} to etc/config.{csh,sh}/settings since these
  should not be changed by the user anyhow.

- Use gcc/g++ as common base and specialize afterwards (to reduce
  duplication)

- adjust format of WM_PROJECT_SITE fallback to allow for easier
  automated edits