Missing SIZE_MAX

Commit 2e70d6b5e added a dependency on SIZE_MAX to libpq/fe_exec.c.
According to C99 and recent POSIX, that symbol should be provided
by <stdint.h>, but SUS v2 (POSIX 2001) doesn't require <stdint.h>
to exist at all ... and I now notice that gaur/pademelon doesn't
have it, and unsurprisingly is failing to compile fe_exec.c.

We have a workaround for that symbol in timezone/private.h:

#ifndef SIZE_MAX
#define SIZE_MAX ((size_t) -1)
#endif

and a bit of grepping finds other places that are using the (size_t) -1
trick explicitly. So what I'm tempted to do is move the above stanza
into c.h. Any objections?

regards, tom lane