Mouse highlight in *compilation* buffers

[Eli Zaretskii]

The mouse-face highlighting of compiler messages is sometimes too
greedy, I think.  For example, if the message text is

  foo.c:5: warning: implicit declaration of function `bar'

then the mouse-face property extends up to and excluding the colon
that is after the "warning" part.  Do we want that, or is the intent
to highlight only the file name and the line number?  I think it's the
latter, because an error message, like this:

  foo.c:11: error: conflicting types for 'bar'

does _not_ have the "error" part highlighted, i.e. mouse-face ends at
the colon after the line number.  So for consistency's sake, the
"warning" part should not be highlighted as well.  (It looks like
"warning" is treated specially due to its appearance in
compilation-error-regexp-alist-alist.)