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".

doc/src/sgml/Makefile

@@ -132,7 +132,7 @@ INSTALL.html: %.html : stylesheet-text.xsl %.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) $(XSLTPROC_HTML_FLAGS) $^ >$@
 
 INSTALL.xml: standalone-install.sgml installation.sgml version.sgml
-	$(OSX) $(SGMLINCLUDE) -x lower $(filter-out version.sgml,$^) >$@.tmp
+	$(OSX) $(SPFLAGS) $(SGMLINCLUDE) -x lower $(filter-out version.sgml,$^) >$@.tmp
 	$(call mangle-xml,chapter)
 
 
@@ -144,7 +144,7 @@ INSTALL.xml: standalone-install.sgml installation.sgml version.sgml
 # if we try to do "make all" in a VPATH build without the explicit
 # $(srcdir) on the postgres.sgml dependency in this rule.  GNU make bug?
 postgres.xml: $(srcdir)/postgres.sgml $(ALLSGML)
-	$(OSX) $(SGMLINCLUDE) -x lower $< >$@.tmp
+	$(OSX) $(SPFLAGS) $(SGMLINCLUDE) -x lower $< >$@.tmp
 	$(call mangle-xml,book)
 
 define mangle-xml