Allow to use pkg-config and friends
For linking to a lib, I would prefer to rely on
pkg-config or stuff like that. Unfortunately, if I use a call to
pkg-config will have to be present each time the module is imported. I would like to be able to specify lazy extra cflags/ldflags: don't evaluate them unless you have to compile the module. Trying to pass some object which lazy evaluate to a list does not work as cffi will evaluate it to get a hash.