qgis sets default_variants incorrectly

[ryandesign (Ryan Carsten Schmidt)]

qgis sets default_variants unconditionally:

$ grep default_variants $(port file qgis) 
default_variants	+python27 +postgis +postgresql91

This causes problems for users who want to use a variant different from one of the defaults:

$ sudo port install qgis +python26
Error: qgis: Variant python26 conflicts with python27
Error: Unable to open port: Error evaluating variants

You must only set +python27 as the default variant if the user has not already requested a different python variant. See wiki:PortfileRecipes#default_variants.

Similarly, you must only set +postgresql91 as the default variant if the user has not already requested a different postgresql variant.

[Veence (Vincent)]

Done in r91416. Thanks Ryan.