Cleanup NAN code in float.h, too.

In the same spirit as 3bf905692, assume that all compilers we still
support provide the NAN macro, and get rid of workarounds for that.

The C standard allows implementations to omit NAN if the underlying
float arithmetic lacks quiet (non-signaling) NaNs.  However, we've
required that feature for years: the workarounds only supported
lack of the macro, not lack of the functionality.  I put in a
compile-time #error if there's no macro, just for clarity.

Also fix up the copies of these functions in ecpglib, and leave
a breadcrumb for the next hacker who touches them.

History of the hacks being removed here can be found in commits
1bc2d544b, 4d17a2146, cec8394b5.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1952095.1759764279@sss.pgh.pa.us

src/interfaces/ecpg/ecpglib/data.c

@@ -69,34 +69,23 @@ garbage_left(enum ARRAY_TYPE isarray, char **scan_length, enum COMPAT_MODE compa
 	return false;
 }
 
-/* stolen code from src/backend/utils/adt/float.c */
-#if defined(WIN32) && !defined(NAN)
-static const uint32 nan[2] = {0xffffffff, 0x7fffffff};
-
-#define NAN (*(const double *) nan)
-#endif
 
+/*
+ * Portability wrappers borrowed from src/include/utils/float.h
+ */
 static double
 get_float8_infinity(void)
 {
-#ifdef INFINITY
 	return (double) INFINITY;
-#else
-	return (double) (HUGE_VAL * HUGE_VAL);
-#endif
 }
 
 static double
 get_float8_nan(void)
 {
-	/* (double) NAN doesn't work on some NetBSD/MIPS releases */
-#if defined(NAN) && !(defined(__NetBSD__) && defined(__mips__))
 	return (double) NAN;
-#else
-	return (double) (0.0 / 0.0);
-#endif
 }
 
+
 static bool
 check_special_value(char *ptr, double *retval, char **endptr)
 {