zz40-xc-ovr.m4: 1.0 interface stabilization

- Stabilization results in 4 public interface m4 macros:
  XC_CONFIGURE_PREAMBLE
  XC_CONFIGURE_PREAMBLE_VER_MAJOR
  XC_CONFIGURE_PREAMBLE_VER_MINOR
  XC_CHECK_PATH_SEPARATOR
- Avoid one level of internal indirection
- Update comments
- Drop XC_OVR_ZZ40 macro