closes#7239
The old behaviour was to set CMAKE_INSTALL_MANDIR to /usr/local/man
when MANPREFIX wasn't defined. This caused mismatching installation
paths when the installation prefix wasn't /usr/local.
This fix explicitely checks that the prefix is /usr/local to change
the value of CMAKE_INSTALL_MANDIR, and uses the default behaviour
otherwise, as /usr/local is the exception rather than the norm
(as per man hier(7)).