Some Unix-like systems (e.g. the BSDs) keep man pages in man/,
others (e.g. Linux) keep man pages in share/man/.
By using CMAKE_INSTALL_MANDIR there's no need to maintain a
list of per-OS locations, and the proper location can be
automatically detected.
Fixes man page installation on NetBSD.
Signed-off-by: Nia Alarie <nia@NetBSD.org>
Signed-off-by: Nia Alarie <nia@NetBSD.org>