PR64747 describes a case where long lines in a c++ source file make
source-highlight run longer than 2m13s.

The problem is that some regexps are unanchored, and consequently run to the
end of the long lines.

Fix this by adding a heuristic that does the rule matching in two phases:
- anchored: add a "\\A[[:blank:]]*" prefix to the regexp.  If the rule
  matching yields a best-possible result, the next phase case be skipped.
- unanchored: use the regexp as is, be that anchored or unanchored.

This brings executing time down to 3s.

Written on top of commit e4cf32d ("Modernize configure.ac") due to PR64764
( https://savannah.gnu.org/bugs/?64764 ).

I've also added lines to all the touched files to specify file local variable
values for emacs to make editing less painful.

Tested on x86_64-linux.  Tested using:
- make check, and
- comparing output for the test-case with and without the patch.

Bug: https://savannah.gnu.org/bugs/?64747