Add -I flag for builddir to find generated spec_config.h.

Also remove the ../spec_config.h target. It didn't work as-is
since it referenced file in the wrong directory. Instead of fixing
this I decided to remove. In normal build, the previous directory
does build it first. In the case of attempting to build specific
directories first, doesn't work yet due to other problems already
(like missing dns components).

This was noticed in a failed distcheck.