UNXSUFFIX was set up by issue 16860 to help with MACOSX but the definition is
hardcoded in scp2 and now there are two flavours of MACOSX. It would be better
for all platforms to take the definition from the solenv/inc/unx*mk files.

Notice this usage:
inc/macros.inc:    #define LIBNAME(name) 
STRING(CONCAT5(lib,name,OFFICEUPD,DLLSUFFIX,UNXSUFFIX))

DLLSUFFIX is passed in on the command line by -DDLLSUFFIX but UNXSUFFIX is hard
coded thus:
#ifdef UNX
    #ifdef MACOSX
        #define UNXSUFFIX .dylib
    #else
        #define UNXSUFFIX .so
    #endif
#endif

But .dylib and .so are already defined in solenv/inc/unx*mk for example:
DLLPOST=.so

By copying how DLLSUFFIX is handled it may be possible to do this:
(a) remove the #define UNXSUFFIX from scp2
(b) patch solenv/inc/pstrules.mk as follows:
--- pstrules.mk 8 Sep 2005 09:38:11 -0000       1.37
+++ pstrules.mk 4 Dec 2005 12:52:44 -0000
@@ -201,7 +201,7 @@
 .IF "$(OS)"=="SOLARIS"
        cpp.lcc -+ -P $(CDEFS) $(SCPDEFS) -DDLLSUFFIX=$(DLLSUFFIX) -I. -I$(INC)
-I$(INCLOCAL) -I$(INCGUI) -I$(INCCOM) $(SOLARINC) $(*:b).scp >
$(MISC)$/{$(subst,$(@:d:d:d), $(@:d:d))}$/$(*:b).pre
 .ELSE
-       cpp.lcc -+ -P $(CDEFS) $(SCPDEFS) -DDLLSUFFIX=$(DLLSUFFIX) -I. -I$(INC)
-I$(INCLOCAL) -I$(INCGUI) -I$(INCCOM) $(SOLARINC) $(*:b).scp >
$(MISC)$/{$(subst,$(@:d:d:d), $(@:d:d))}$/$(*:b).pre
+       cpp.lcc -+ -P $(CDEFS) $(SCPDEFS) -DDLLSUFFIX=$(DLLSUFFIX)
-DUNXSUFFIX=$(DLLPOST) -I. -I$(INC) -I$(INCLOCAL) -I$(INCGUI) -I$(INCCOM)
$(SOLARINC) $(*:b).scp > $(MISC)$/{$(subst,$(@:d:d:d), $(@:d:d))}$/$(*:b).pre
 .ENDIF
 .ENDIF
 .IF "$(common_build_srs)"!=""

But need to check if this has any side effects, and have all platforms set
DLLPOST correctly?