Allow setting manpage install dir
Allow setting manpage install dir

Helps with packaging for systems where manpages are not installed under
CMAKE_INSTALL_PREFIX.
e.g. on Arch, CMAKE_INSTALL_PREFIX should be /usr, but manpages are in
/usr/share/man rather than /usr/man.

tomhrr/dale,jtsymon/dale,tomhrr/dale,jtsymon/dale,jtsymon/dale,tomhrr/dale,jtsymon/dale,tomhrr/dale