Commit 7bff56a0 worked in an incremental build, but fails for a
fresh clone; apparently, if make sees both an actual file
spelling and an inference rule, only the exact spelling is used.

  CCLD   libvirt_driver_test.la
  CC     libvirt_driver_remote_la-remote_driver.lo
remote/remote_driver.c:4707:34: fatal error: remote_client_bodies.h: No such file or directory
compilation terminated.

BUILT_SOURCES to the rescue, instead of trying to mess with .lo
dependencies directly.

* src/Makefile.am (REMOTE_DRIVER_PREREQS, %remote_driver.lo): Drop...
(BUILT_SOURCES): ...and add here instead.