Add an ifneq to avoid make warning on AIX --- there is a separate rule

for postgres: on line 86, and line 43 shouldn't be used.  Noted while
looking at kookaburra buildfarm results.

src/backend/Makefile

@@ -4,7 +4,7 @@
 #
 # Copyright (c) 1994, Regents of the University of California
 #
-# $PostgreSQL: pgsql/src/backend/Makefile,v 1.109 2005/07/28 04:31:30 tgl Exp $
+# $PostgreSQL: pgsql/src/backend/Makefile,v 1.110 2005/10/27 20:45:29 tgl Exp $
 #
 #-------------------------------------------------------------------------
 
@@ -38,14 +38,17 @@ all: submake-libpgport postgres $(POSTGRES_IMP)
 
 ifneq ($(PORTNAME), cygwin)
 ifneq ($(PORTNAME), win32)
+ifneq ($(PORTNAME), aix)
 
 postgres: $(OBJS)
 	$(CC) $(CFLAGS) $(LDFLAGS) $(export_dynamic) $^ $(LIBS) -o $@
 
 endif
 endif
+endif
 
 ifeq ($(PORTNAME), cygwin)
+
 postgres: $(OBJS) $(DLLINIT) postgres.def libpostgres.a
 	$(DLLTOOL) --dllname $@$(X) --output-exp $@.exp --def postgres.def
 	$(CC) $(CFLAGS) $(LDFLAGS) -o $@$(X) -Wl,--base-file,$@.base $@.exp $(OBJS) $(LIBS)
@@ -78,7 +81,6 @@ libpostgres.a: postgres.def
 
 endif # win32
 
-
 ifeq ($(PORTNAME), aix)
 
 postgres: $(POSTGRES_IMP)