bug#32322: 27.0.50; Connect flymake to next-error-function when enabled

[Terje Larsen <terlar@gmail.com>]

I know that some other packages utilize the 'next-error-function' to get
support for 'next-error' and 'previous-error'. I wonder if it would make
sense to set this up by default when the flymake mode is enabled. I have
currently enabled it by:

  (setq next-error-function 'flymake-goto-next-error)

And it seems to work flawlessly.

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin13.4.0, NS
appkit-1265.21 Version 10.9.5 (Build 13F1911))
of 2018-05-07 built on builder10-9.local
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.6

Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

Important settings:
  value of $LANG: en_SE.UTF-8
  locale-coding-system: utf-8

Major mode: Emacs-Lisp

-- 
// Terje Larsen