dependency version specification should support ranges

[boegel]

Specifying dependency versions should have support for ranges, such that build specifications can be made more flexible, e.g.:

dependencies = [
    ('CMake, '>= 2.4'),
    ('Qt'),  # no explicit version, any version will do
]

This should probably be tackled in easyconfig format 2.x, rather than 1.0.

[stdweird]

@boegel i don't believe there is anything where you can have a freefloat dependency; and if it is the case, you can always use > 0. i would make the range mandatory.

[boegel]

@stdweird: Yes, I doubt no specifying any version (range) at all makes makes much sense, and if we don't support it it's not like it's a limitation.