py/mkrules.mk: Fix auto-qstr generation when "make -B" is used.

dpgeorge · dpgeorge · commit 85f7b0b468ce · 2017-05-26T13:12:42.000+10:00
For make v3.81, using "make -B" can set $? to empty and in this case the auto-qstr generation needs to pass all args (ie $^) to cpp. The previous fix for this (which was removed in 23a693e) used if statements in the shell command, which gave very long lines that didn't work on certain systems (eg cygwin). The fix in this patch is to use an $if(...) expression, which will evaluate to $? (only newer prerequisites) if it's non empty, otherwise it will use $^ (all prerequisites).
diff --git a/py/mkrules.mk b/py/mkrules.mk
@@ -71,7 +71,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
 
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) | $(HEADER_BUILD)/mpversion.h
 	$(ECHO) "GEN $@"
-	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $? >$(HEADER_BUILD)/qstr.i.last;
+	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $(if $?,$?,$^) >$(HEADER_BUILD)/qstr.i.last;
 
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"
