zz40-xc-ovr.m4: truly do version conditional overriding

- version conditional overriding
- catch unexpanded XC macros
- fix double words in comments
zz40-xc-ovr.m4: emit witness message in configure BODY

This avoids witness message in output when running configure --help,
while sending the message to config.log for other configure runs.
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
zz40-xc-ovr.m4: check another three basic utilities
zz40-xc-ovr.m4: fix 'wc' detection

- Take into account that 'wc' may return leading spaces.

- Set internationalization behavior variables.

Tor Arntsen analyzed and reported the issue.

URL: http://curl.haxx.se/mail/lib-2013-01/0351.html
zz40-xc-ovr.m4: fix 'wc' detection - follow-up

- Take into account that 'wc' may return leading spaces and/or tabs.

- Set initial IFS to space, tab and newline.
zz40-xc-ovr.m4: fix 'wc' detection - follow-up 2

- Fix a pair of single quotes to double quotes.

URL: http://curl.haxx.se/mail/lib-2013-01/0355.html
Reported by: Tor Arntsen