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

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 23a693ec2d)
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).

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 $@"