Append project compiler arguments after arguments from build system anddisable warning as error

Summary:
Arguments from project option were not applied or applied
incorrectly since D7752. This patch fixes sime problem when setting
custom arguments to the parser.

Also, since we take arguments from the build system, warning as error
became effective on the parser. I added Wno-error at the end of the
default parser arguments.

Reviewers: KDevelop, kfunk, apol

Reviewed By: KDevelop, kfunk

Subscribers: mwolff, brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7876