Remove build-time generated files when "make clean" is run.

Remove build-time generated files when "make clean" is run.

"make clean" should remove all files generated by building a program, while
"make distclean" should also remove files generated by configuring the
program.  Previously some generated files during the build process, such
as man pages, were left behind when "make clean" was run.  This commit
only leaves configuration files after "make clean" is run, and removes
all other generated files.

Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>