Fix another portability bug in recent pgbench commit.

Commit 547f04e7 produced errors on AIX/xlc while building plpython.  The
new code appears to be incompatible with the hack installed by commit
a11cf433.  Without access to an AIX system to check, my guess is that
_POSIX_C_SOURCE may be required for <time.h> to declare the things the
header needs to see, but plpython.h undefines it.

For now, to unbreak build farm animal hoverfly, just move the new
pg_time_usec_t support into pgbench.c.  Perhaps later we could figure
out what to rearrange to put it back into a header for wider use.

Discussion: https://postgr.es/m/CA%2BhUKG%2BP%2BjcD%3Dx9%2BagyTdWtjpOT64MYiGic%2Bcbu_TD8CV%3D6A3w%40mail.gmail.com

src/bin/pgbench/pgbench.c

@@ -320,6 +320,13 @@ typedef struct SimpleStats
 	double		sum2;			/* sum of squared values */
 } SimpleStats;
 
+/*
+ * The instr_time type is expensive when dealing with time arithmetic.  Define
+ * a type to hold microseconds instead.  Type int64 is good enough for about
+ * 584500 years.
+ */
+typedef int64 pg_time_usec_t;
+
 /*
  * Data structure to hold various statistics: per-thread and per-script stats
  * are maintained and merged together.
@@ -658,6 +665,24 @@ static const PsqlScanCallbacks pgbench_callbacks = {
 	NULL,						/* don't need get_variable functionality */
 };
 
+static inline pg_time_usec_t
+pg_time_now(void)
+{
+	instr_time	now;
+
+	INSTR_TIME_SET_CURRENT(now);
+
+	return (pg_time_usec_t) INSTR_TIME_GET_MICROSEC(now);
+}
+
+static inline void
+pg_time_now_lazy(pg_time_usec_t *now)
+{
+	if ((*now) == 0)
+		(*now) = pg_time_now();
+}
+
+#define PG_TIME_GET_DOUBLE(t) (0.000001 * (t))
 
 static void
 usage(void)