POSIX make(1) does not support prerequisites on suffix rules.

POSIX make(1) does not support prerequisites on suffix rules.
For HTML generation, the "mandoc" prerequisite isn't needed anyway
because ${WWW_MANS} already explicitly depends on mandoc.
Issue reported by Sevan Janiyan and Leah Neukirchen.

This is not critical for release because it is only used for
a maintainer target.  While here, i also fixed the associated
shell command to use the freshly built mandoc binary rather
than whatever may be in the $PATH.