Avoid defining __pure2 if already defined

This gets rid of a warning on Mac OS X due to the fact that
sys/cdefs.h defines it already.

src/bsd_cdefs.h

@@ -93,7 +93,7 @@
  * for a given compiler, let the compile fail if it is told to use
  * a feature that we cannot live without.
  */
-#if __GNUC_PREREQ__(2, 7) || defined(__INTEL_COMPILER)
+#if !defined(__pure2) && (__GNUC_PREREQ__(2, 7) || defined(__INTEL_COMPILER))
 #define	__pure2		__attribute__((__const__))
 #endif