* Add a general "install-headers" support to the bsd.*.mk files
	     so that "make install-headers" would descend the source dirs
	     and install headers in each header that has a install-header
	     target. 

This would probably be okay, although I would suggest following common
practice and calling the target `includes'.

A third approach would be to move the .h files into /usr/src/include,
which is arguable where they belong.