As noticed by david@, empty files were installed instead of formatted

GL manuals because Imake still tried to use nroff(1), tbl(1), and eqn(1)
which do not exist in base any longer.  Thus, let the Xenocara build use
mandoc(1) instead, which has native tbl(7) support, and passes eqn source
code through unchanged for now, which is more useful than what groff did.
However, as suggested by espie@, use the groff port when building Ports.
That always works because all ports using imake still have USE_GROFF=Yes.

thanks for useful hints to todd@ matthieu@ espie@ naddy@
tested by todd@ in Xenocara builds on i386, amd64, and macppc
ok matthieu@ and naddy@, and miod@ also agrees with the general direction

util/cf/OpenBSD.cf

@@ -244,9 +244,22 @@ XCOMM operating system:  OSName (OSMajorVersion./**/OSMinorVersion./**/OSTeenyVe
 /*
  * Documentation formatting
  */
-#define NeqnCmd			neqn -Tascii
-#define NroffCmd		nroff -Tascii
-#define	EqnCmd 			eqn -Tps
+#ifdef Ports
+# define NroffCmd		/usr/local/bin/nroff -Tascii
+# define TroffCmd		/usr/local/bin/nroff -Tps
+# define HTMLroffCmd		/usr/local/bin/nroff -Thtml
+# define NeqnCmd		/usr/local/bin/neqn -Tascii
+# define EqnCmd			/usr/local/bin/eqn -Tps
+# define TblCmd			/usr/local/bin/tbl
+#else
+# define NroffCmd		mandoc
+# define TroffCmd		mandoc -Tps
+# define HTMLroffCmd		mandoc -Thtml
+# define NeqnCmd		cat
+# define EqnCmd			cat
+# define TblCmd			cat
+#endif
+
 #define HasGroff		YES
 
 #ifndef HasCookieMaker