Add a MANPAGES knob to devel/valgrind{-devel}

devel/valgrind/Makefile
devel/valgrind/pkg-plist
devel/valgrind-devel/Makefile
devel/valgrind-devel/pkg-plist
Add a MANPAGES option. valgrind's man pages are built with
docbook-xml, which has lots of dependencies. Disabling MANPAGES will
speed the build.

Reviewed by: bdrewery, brd
Approved by: bdrewery (ports, maintainer)
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6667