Further refine some #ifs:

- Simplify the logic by using __GNUC_PREREQ__.
  Suggested by stefanf.
- Make math.h compile with old (pre-8.0) versions of icc.
  Submitted by sf [sic].

lib/msun/src/math.h

@@ -34,11 +34,11 @@ extern const union __nan_un {
 	float		__uf;
 } __nan;
 
-#if (defined(__GNUC__) && ((__GNUC__ >= 3 && __GNUC_MINOR__ >= 3) || __GNUC__ >= 4)) || defined(__INTEL_COMPILER)
+#if __GNUC_PREREQ__(3, 3) || (defined(__INTEL_COMPILER) && __INTEL_COMPILER >= 800)
 #define	__MATH_BUILTIN_CONSTANTS
 #endif
 
-#if (defined(__GNUC__) && __GNUC__ >= 3 && !defined(__INTEL_COMPILER))
+#if __GNUC_PREREQ__(3, 0) && !defined(__INTEL_COMPILER)
 #define	__MATH_BUILTIN_RELOPS
 #endif