GNU License warning:

[Chris Wright via Digitalmars-d]

You offer an API and someone decides to build on it using the GPL -- no 
trouble there; your work is not a derivative of theirs, so their 
copyright cannot place restrictions on your work.

You build against an open standard and the only implementation is GPL -- 
your work is a derivative of the standard, not necessarily the GPL'd work.

You build against an open standard with an MIT licensed implementation  
and someone else builds a GPL implementation -- no trouble there; your 
work is not a derivative of theirs, so their copyright cannot place 
restrictions on your work.