cmake: tweak manpage install location

cmake: tweak manpage install location

This patch is needed when wanting to install binaries/libraries to other
locations than data. The linux distro Exherbo installs binaries and
libraries to /usr/<chost>/bin, /usr/<chost>/lib respectively but
manpages should still go in /usr/share/man/ because they are
architecture independent, without this patch they go in
/usr/<chost>/share/man unconditionally.

cmake documentation states:
DATAROOTDIR - read-only architecture-independent data root (share)

So this patch makes it use that so that its configurable.