Fix doc/Makefile dependency for install target

[Xiami2012]

This fixes GitHub issue #102
by making man document files require mandirs

For explicit rules, multiple rules for one target
results a merge of prerequisites [1]:

    example : preq-1
    example : | preq-3
    example : preq-2
        recipes

    <==> (equals)

    example : preq-1 preq-2 | preq-3
        recipes

For implicit rules, that is not:

    %.o : %.h
    %.o : %.c
        recipes

    <==>

    %.o : %.c
        recipes

    "%.o : %.h" w/o recipes means a Cancel Rule in implicit rule [2]
    which removes the specified implicit rule

[1]: https://www.gnu.org/software/make/manual/make.html#Multiple-Rules
[2]: https://www.gnu.org/software/make/manual/make.html#Canceling-Rules

[superm1]

Thanks!