Warning as error screws up highlighting

[Guillaume Racicot]

Hi KDevelop developpers,

While testing the latest kdevelop, the highlighting of my project
really screwed up. Hardly a single line was highlighted properly.

I noticed that the problem arises from the presence of the `-Werror`
being picked up by the parser. This is since
https://phabricator.kde.org/D7752 has been accepted.

This recent feature can really screws up highlighting. I have thought
of some solutions for this:

 - Having a whitelist of warnings flags and filter out other warning arguments.
 - Remove `-Werror` from extra arguments.
 - Remove all warnings related extra arguments.

The second solution is indeed the fastest to implement, and wont
influence other warning options. and due to the approaching 5.2
release, it might be appealing. The third is a bit extreme, but
ensures no warning settings from the build system will influence
parsing. The first might be great, but would require more
implementation efforts.

I'm willing to send a patch, but I would like to have your feedback
about what to do before sending it.

Thanks and keep up the good work!