doc: Add SPFLAGS to osx calls

doc: Add SPFLAGS to osx calls

This enables the same OpenSP warnings on osx calls that we get from
onsgmls (make check) and formerly from openjade.

Older tool chains apparently have some of these warnings on by
default (see comment at SPFLAGS assignment).  So users of such tool
chains would complain about warnings or errors that users of newer tool
chains would not see, unless they used "make check".