Improve automated testing rules to allow other test frameworks

- make runtests a double-colon rule, so that other test frameworks
  can add their own recipes independently
- only define runtests:: $TESTSCRIPTS rule when there are TESTSCRIPTS
  (to avoid having it run every time when no TESTSCRIPTS are defined)
- $(strip $TAPFILES) inside ifneq to fix trouble when TAPFILES=' '

configure/RULES_BUILD

@@ -335,10 +335,12 @@ $(MODNAME): %$(MODEXT): %$(EXE)
 #---------------------------------------------------------------
 # Automated testing
 
-runtests: $(TESTSCRIPTS)
+ifneq ($(strip $(TESTSCRIPTS)),)
+runtests:: $(TESTSCRIPTS)
 ifdef RUNTESTS_ENABLED
 	-$(PERL) -MTest::Harness -e 'runtests @ARGV if @ARGV;' $^
 endif
+endif
 
 testspec: $(TESTSCRIPTS)
 	@$(RM) $@
@@ -349,7 +351,7 @@ testspec: $(TESTSCRIPTS)
 	$(if $(TESTSPEC_$(OS_CLASS)), @echo "Harness: $(TESTSPEC_$(OS_CLASS))" >> $@)
 
 test-results: tapfiles
-ifneq ($(TAPFILES),)
+ifneq ($(strip $(TAPFILES)),)
 ifdef RUNTESTS_ENABLED
 	$(PROVE) --failures --ext .tap --exec "$(CAT)" --color $(TAPFILES)
 endif