Compile issues

[Dave Love]

next-error is currently broken.  With the following (modulo
indentation) in *compilation*, you get `Moved past last error' on the
first C-x `.

  cd /tmp/      # -*-compilation-*-
  Entering directory `/tmp/'
  cc a.c
  a.c:2: error: syntax error before "foo"
  a.c:2: warning: data definition has no type or storage class
  
  Compilation exited abnormally with code 1 at Thu Mar 25 12:12:26

Please revert the (undocumented) change which makes next-error's arg
non-optional.  This breaks Erlang support, at least.

NEWS reports that `by default' warning messages are skipped, but gives
no indication of how to get the old behaviour back.  I think the old
behaviour should be the default.

`compilation-error-regexp-alist' is now apparently incompatible with
the old version, but users or external packages are likely to rebind
it.  This change isn't documented, and it's not clear to me what you
should do instead.

Also, `compile-internal' has apparently changed incompatibly and been
declared obsolete, but that isn't in NEWS either, and it's commonly
used by add-ons (following what various bundled packages did).

Please bear in mind generally people who use compile externally.

By the way, it seems to me that several `a-zA-Z's in the compile.el
regexps need to be changed to [:alpha:] (or [:alnum:]).