I have a project, a big portion of which can be released as LGPL, but there are still some portions that must keep a more restrictive license. I'm not sure what exactly is allowed. The intent is distributing the whole package with source code, but allow redistribution and modification beyond personal use only for the LGPL portions.
Can a proprietary program link to an LGPL library? Yes, that's the main point of LGPL, I understand.
Can the link be static or dynamic? Yes to dynamic, for static linking a way to link to a later version must be given. I assume this is subject to "later versions" being compatible, and providing the source code (and necessary build elements) is enough, even if the source code is not redistributable.
Can an LGPL program link (dynamically or statically) to a non-free library? In this case it would be an "optional" feature, a way to get some enhanced functionality, but the core of the program would work fine without the library.
Can the build system for the LGPL code dynamically (and optionally) fetch non-free code and compile it? Say I distribute the LGPL source code, everyone is free to use it, compile it and redistribute it; but those with an "extended" license get access to additional non-free code which they can compile together with the LGPL portions, and they are not allowed to redistribute this part. Would that be fine?
Assuming the authors and copyright holders agree, can the LGPL license be modified to add an exemption that allows linking to other proprietary portions of the project?