Handle special case that $manpath is undefined.

The code

  MANPATH="$man_dir:$(manpath)"

led to problems on opensolaris systems, as $manpath is not defined there. The following code works

  manpath=${manpath:-}
  MANPATH="$man_dir:${manpath}"

The first line sets an empty default value. Note also that the () brackets have been replaced by curly brackets.