configure: only check dispatch header on darwin

Fixes build of lavd/jack on linux if dispatch happens to be available on
this platform. dispatch, as well as its dependencies kqueue and pwq are
generally not installed / distribued on linux systems. If it happens to
be the case, you want to explicitely link against the libraries (using
-ldispatch) as opposed to darwin where it is part of the standard
library and -ldispatch doesn't work.
