Update to 2.18.7.

This minor update from QGIS 2.18.5 to 2.18.7 comes with some new features and
many bugfixes. Especially, many problems with SAGA GIS processing seem to be
solved.

Port changes:

• Update to 2.18.7.
• Remove CMAKE_ARGS which do not exist.
• Add new python dependencies cycler, dateutil, and functools32.
• Getting rid of special handlings for 9.x.
• Move USES=compiler:features into main section.
• Update pkg-plist.

PR: 218878
Submitted by: Rainer Hurling <rhurlin@gwdg.de> (maintainer)