The following code :

	#define hash_hash # ## #
	#define mkstr(a) # a
	#define in_between(a) mkstr(a)
	#define join(c, d) in_between(c hash_hash d)

	char p[] = join(x, y);

should produce :

	char p[] = "x ## y";

instead of :

	char p[] = "xy";

I think stringify() is called too many. That's why I propose this fix.
However, even though it fixes this problem, I cannot tell you of any collateral damages ;)

Phil;