Add, optional, support for 128bit integers.

We will, for the foreseeable future, not expose 128 bit datatypes to
SQL. But being able to use 128bit math will allow us, in a later patch,
to use 128bit accumulators for some aggregates; leading to noticeable
speedups over using numeric.

So far we only detect a gcc/clang extension that supports 128bit math,
but no 128bit literals, and no *printf support. We might want to expand
this in the future to further compilers; if there are any that that
provide similar support.

Discussion: 544BB5F1.50709@proxel.se
Author: Andreas Karlsson, with significant editorializing by me
Reviewed-By: Peter Geoghegan, Oskari Saarenmaa

src/include/c.h

@@ -293,6 +293,17 @@ typedef unsigned long long int uint64;
 #define HAVE_INT64_TIMESTAMP
 #endif
 
+/*
+ * 128-bit signed and unsigned integers
+ *		There currently is only a limited support for the type. E.g. 128bit
+ *		literals and snprintf are not supported; but math is.
+ */
+#if defined(PG_INT128_TYPE)
+#define HAVE_INT128
+typedef PG_INT128_TYPE int128;
+typedef unsigned PG_INT128_TYPE uint128;
+#endif
+
 /* sig_atomic_t is required by ANSI C, but may be missing on old platforms */
 #ifndef HAVE_SIG_ATOMIC_T
 typedef int sig_atomic_t;