Avoid using `@` in macro substitution that confuses FreeBSD make

```
making mjit_config.h
make: Unknown modifier ' '
make: Unclosed variable specification (expecting ')') for "Q" (value "") modifier
/bin/sh: Syntax error: end of file unexpected (expecting "}")
*** Error code 2

Stop.
```

The at sign probably conflicts the `:@temp@string@` modifier syntax
supported by FreeBSD make.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62414 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Makefile.in

@@ -538,7 +538,7 @@ un-runnable:
 mjit_config.h:
 	$(ECHO) making $@
 	@{ \
-	$(Q:@=:) set -x; \
+	test "$(Q)" = @ || set -x; \
 	echo '#ifndef RUBY_MJIT_CONFIG_H'; \
 	echo '#define RUBY_MJIT_CONFIG_H 1'; \
 	\