build-sys: Add --disable-manpages option

build-sys: Add --disable-manpages option

1) Embedded systems often don't want man pages on the
   target; rather than pointlessly building them, then ignoring
   the result, allow just not building them at all
2) When bootstrapping an operating systems, documentation is the
   source of many cyclical dependencies, and allowing it to
   be explicitly disabled is useful for earlier build passes.