Make OMake support slightly less expensive (bug#39595)

Make OMake support slightly less expensive (bug#39595)

When run with -p or -P, OMake regurgitates error messages that
prevented further progress, indented by 6 spaces.  Use that fact
to ameliorate the modification done to other error message regexps.

* lisp/progmodes/compile.el (compilation-parse-errors):
When 'omake' is enabled, allow error messages to be indented by 0 or 6
spaces instead of any number of spaces, to avoid pathological
behaviour.
(compilation-error-regexp-alist-alist): Anchor the 'omake' pattern to
bol for performance.  Repair the 'ruby-Test::Unit' pattern, which
relied on the previously over-generous 'omake' hack.
* etc/compilation.txt (OMake): Add examples.
* test/lisp/progmodes/compile-tests.el (compile-tests--test-regexps-data)
(compile-test-error-regexps): Add test for OMake (indented error).