This can be used to specify an alternate source file name for an
individual manpage. For example, the following snippet:
MAN= foo::bar.3 MANSRC.foo::bar.3= foo__bar.3
Will install a foo::bar.3 manpage but use foo__bar.3 as the source
input to MANFILTER, etc.
Suggested by: ivy
Sponsored by: Chelsio Communications