Makefile: Use a variable for all commands

This is a little more verbose in places but it lets folks have these
tools in non-standard locations.  This also cleans up the uninstall
target so that every command is on its own line.

Signed-off-by: David Aguilar <davvid@gmail.com>
Makefile: Export DESTDIR

Set DESTDIR in the environment to ensure that the documentation sub-
Makefiles honor the specified DESTDIR.

Signed-off-by: David Aguilar <davvid@gmail.com>