If CapnProto is part of cmake build, macro CAPNP_GENERATE_CPP
uses in-build capnp tools.

This change fixes `make check` target with cmake.

All tests pass except
1: [ FAIL ] exception-test.c++:30: legacy test:
    Exception/TrimSourceFilename
1: /home/brano/src/capnproto/c++/src/kj/exception-test.c++:31:
    failed: expected (trimSourceFilename(
    "/home/brano/src/capnproto/c++/src/kj/exception-test.c++"))
    == ("kj/exception-test.c++");

Not sure how to fix the test without disabling it.