GNUInstallDirs: Update CMAKE_INSTALL_MANDIR for FreeBSD
CMAKE_INSTALL_MANDIR is set to
man for *BSD. However, for FreeBSD it's no longer correct, as the canonical location was changed to
share/man more than a year ago:
needs an exception for FreeBSD (or more explicit pattern like
(NetBSD|OpenBSD|BSD|DragonFly), but I'm not sure I know all actual *BSD names to list).