Inject $(ICU_LIBS) regardless of platform.

It appeared in a conditional that excludes AIX, Cygwin and MinGW.  Give
ICU support a chance to work on those platforms.  Back-patch to v10,
where ICU support was introduced.

src/backend/Makefile

@@ -39,8 +39,8 @@ OBJS = $(SUBDIROBJS) $(LOCALOBJS) $(top_builddir)/src/port/libpgport_srv.a \
        $(top_builddir)/src/common/libpgcommon_srv.a
 
 # We put libpgport and libpgcommon into OBJS, so remove it from LIBS; also add
-# libldap
+# libldap and ICU
-LIBS := $(filter-out -lpgport -lpgcommon, $(LIBS)) $(LDAP_LIBS_BE)
+LIBS := $(filter-out -lpgport -lpgcommon, $(LIBS)) $(LDAP_LIBS_BE) $(ICU_LIBS)
 
 # The backend doesn't need everything that's in LIBS, however
 LIBS := $(filter-out -lz -lreadline -ledit -ltermcap -lncurses -lcurses, $(LIBS))
@@ -58,7 +58,7 @@ ifneq ($(PORTNAME), win32)
 ifneq ($(PORTNAME), aix)
 
 postgres: $(OBJS)
-	$(CC) $(CFLAGS) $(LDFLAGS) $(LDFLAGS_EX) $(export_dynamic) $(call expand_subsys,$^) $(LIBS) $(ICU_LIBS) -o $@
+	$(CC) $(CFLAGS) $(LDFLAGS) $(LDFLAGS_EX) $(export_dynamic) $(call expand_subsys,$^) $(LIBS) -o $@
 
 endif
 endif