'configflags' ... has non-unique elements

Trying out the ‘qt58’ part, snapcraft complains:

$ snapcraft
Issue while loading plugin: properties failed to load for qt58: The 'configflags' property does not match the required schema: [...] has non-unique elements

The reason for this is that the part tries to pass multiple -skip arguments, which should be followed by a module name. This is impossible today, as snapcraft seems to require the configlags property to have unique entries.

Should snapcraft really impose that restriction?

This is more specifically the autotools plugin demanding that options be unique, see snapcraft/plugins/autotools.py:56 and the unit test respectively checks that snapcraft/tests/plugins/test_autotools.py:59 - I think that can just be dropped.

Bonus points for covering this in the integration test, although I wouldn’t expect that in the same PR addressing this since it’s pretty lacking in general.