abuild can't find pkgconfig (pc:) dependencies between subpackages of the same package.
When compiling mongo-driver-c the libbson, libbson-static and libbson-dev subpackages are also created alongside mongo-c-driver-dev and mongo-c-driver-static.
mongo-c-driver-dev's pkgconfig file has a Requires: libbson-1.0
but the pc:libbson-1.0 dependency isn't found because the file is only present in another subpackage.