add support for optional (OS) dependencies

[boegel]

in some cases, a single easyconfig file should be able to specify that some dependencies should either be provided by the OS, or by modules built by EasyBuild; examples include Qt and wxWidgets, that may very much depend on other OS packages (e.g. X libraries)

the user should be able/forced to specify command line which one of both options should be followed, and the archived easyconfig should clearly specify which option was used, to try and ensure that the build can reproduced exactly

this should problaby be tackled in easyconfig format 2.x (rather than 1.0)

[fgeorgatos]

fyi.

this looks to me as duplicate of #174;
at the time the discussion was about tcsh and "meta-modules"; the point was the same.
Interestingly we were also discussing about ranges, under the same issue and a few more things.

[boegel]

@fgeorgatos: I'll keep this open until it is resolved effectively, since #174 is about a lot more.