Add "nolink" dependency type for shared libraries dependent on shared libraries

[ethouris]

In contradiction to a program, a shared library need not have all symbols resolved even potentially at link time. Therefore, if a library is being created and then used as a dependency then the following specification should be available:

• -depends somelib - link against the current available specification. If both static and dynamic are available, prefer static.
• -depends somelib/static - enforce linking against static library (error if not provided)
• -depends somelib/shared - enforce linking against shared library (error if not provided)
• -depends somelib/virtual - add all sources defined for somelib directly to this target sources instead of linking
• -depends somelib/nolink - enforce target dependency, but don't link against this library at all

The nolink specification should be allowed also when defining a program. This is useful if you have a shared library that will be load and resolved manually, e.g. using dlopen and dlsym functions.