Fix compilation by including cdefs-compat.h in math_private.h

Also fix redefinition of __pure2

src/math_private.h

@@ -17,6 +17,7 @@
 #ifndef _MATH_PRIVATE_H_
 #define	_MATH_PRIVATE_H_
 
+#include "cdefs-compat.h"
 #include "types-compat.h"
 #include "fpmath.h"
 #include <complex.h>

src/openlibm.h

@@ -27,7 +27,9 @@
 #define LONG_DOUBLE
 #endif
 
+#ifndef __pure2
 #define __pure2
+#endif
 
 /*
  * ANSI/POSIX