py/mkrules.mk: Fix Bashism.

py/mkrules.mk

@@ -83,7 +83,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
 
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) | $(HEADER_BUILD)/mpversion.h
 	$(ECHO) "GEN $@"
-	$(Q)if [ "$?" == "" ]; then \
+	$(Q)if [ "$?" = "" ]; then \
 	    echo "QSTR Looks like -B used, trying to emulate"; \
 	    $(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $^ >$(HEADER_BUILD)/qstr.i.last; \
 	else \