It looks like pkg-config silently drops -I and -L flags pointing to
default directories:

```
$ cat /usr/lib/pkgconfig/capnp.pc
prefix=/usr
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

Name: Cap'n Proto
Description: Insanely fast serialization system
Version: 0.7-dev
Libs: -L${libdir} -lcapnp -pthread  -lpthread
Libs.private:  -lpthread
Requires: kj = 0.7-dev
Cflags: -I${includedir} -pthread

$ pkg-config --libs capnp
-lcapnp -pthread -lpthread -lkj -pthread -lpthread
```

Ideally, however, we should use FindPkgConfig.cmake's own facilities
to generate IMPORTED targets from pkg-config files.