Ensure SIZE_MAX can be used throughout our code.

Pre-C99 platforms may lack <stdint.h> and thereby SIZE_MAX.  We have
a couple of places using the hack "(size_t) -1" as a fallback, but
it wasn't universally available; which means the code added in commit
2e70d6b5e fails to compile everywhere.  Move that hack to c.h so that
we can rely on having SIZE_MAX everywhere.

Per discussion, it'd be a good idea to make the macro's value safe
for use in #if-tests, but that will take a bit more work.  This is
just a quick expedient to get the buildfarm green again.

Back-patch to all supported branches, like the previous commit.

Discussion: https://postgr.es/m/15883.1504278595@sss.pgh.pa.us

src/include/c.h

@@ -343,6 +343,11 @@ typedef unsigned PG_INT128_TYPE uint128;
 #define PG_INT64_MAX	INT64CONST(0x7FFFFFFFFFFFFFFF)
 #define PG_UINT64_MAX	UINT64CONST(0xFFFFFFFFFFFFFFFF)
 
+/* Max value of size_t might also be missing if we don't have stdint.h */
+#ifndef SIZE_MAX
+#define SIZE_MAX ((size_t) -1)
+#endif
+
 /*
  * We now always use int64 timestamps, but keep this symbol defined for the
  * benefit of external code that might test it.