The FreeType library is released under a dual license: you can choose either the FTL (FreeType License) or the GPL, version 2. That means that a software stack that uses FreeType has to be license-compatible with one or the other (or both). In cases where the rest of the software stack also allows choices, this can be slightly complex. You end up with a combinatorial explosion of licenses -- in theory, if not in practice.
But why is a choice necessary, anyway?
At issue is the "attribution clause". This is common in the BSD family of licenses: the clauses usually say that you must attribute or credit the authors in the documentation accompanying the (binary distribution of the) software. Let's look at the attribution clause of the FTL itself:
Redistribution in binary form must provide a disclaimer that states that the software is based in part of the work of the FreeType Team, in the distribution documentation. We also encourage you to put an URL to the FreeType web page in your documentation, though this isn't mandatory.
OK, that's fine. You can do that, by adding exactly that to your documentation. It's not an onerous licensing requirement. But it is part of the license and must remain part of the license. This collides with the GPL version 2 because that license demands that the entire derivative work be licensed as GPL version 2 -- that's clause 2b, and the explanation in clause 2, and clause 6. Clause 6 in particular adds "You may not impose any further restrictions on the recipients' exercise of the rights granted herein." And the attribution clause is exactly a restriction that is not allowed by the GPLv2: the license demands that you put something in the documentation, the GPL version 2 does not allow you to add that demand to the license of the whole, end of story.
Well, it would be end of story except that two efforts in the realm of license compatibility have happened: FreeType itself is also available under the GPL version 2, so if you really want to you can apply the GPLv2 to your program and be done with it (this is good for Free Software, but one could argue that it drops the permissiveness of the BSD-style license because now you can't produce a proprietary, binary version and satisfy the license through attribution). And on the GPL side, the GPL version 3 has clause 7b which specifically allows distribution under the GPL version 3 with an additional restriction of the form
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it;
This is just called an additional restriction because that's the language of the GPL. Attribution in the documentation is, like I said earlier, hardly an onerous restriction. Well .. maybe. That's something we'll come to some other day. In any case, it means you can write your software under the terms of the GPL version 3, then add FreeType to it under the terms of the FTL and release the whole under the GPL version 3 with an additional restriction.
To return to my earlier consideration of KDE license policies: there's nothing there that makes it impossible to have GPLv3-or-later code in there license-wise, because you can make compatible choices. But that's a very small sample, and KDE policy remains otherwise.