math: isnan is a C99 feature

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Henning Heinold <heinold@inf.fu-berlin.de>

libc/sysdeps/linux/common/bits/mathcalls.h

@@ -235,15 +235,14 @@ __MATHCALLX (nan,, (__const char *__tagb), (__const__))
 __END_NAMESPACE_C99
 #endif
 
-
-/* Return nonzero if VALUE is not a number.  */
-/* DELETE? __MATHDECL_PRIV adds another "__": so this is "____isnan"???! */
-/*__MATHDECL_PRIV (int,__isnan,, (_Mdouble_ __value), (__const__))*/
-
-#if defined __USE_MISC || defined __USE_XOPEN
+#if defined __USE_MISC || defined __USE_XOPEN || defined __USE_ISOC99
 /* Return nonzero if VALUE is not a number.  */
+__BEGIN_NAMESPACE_C99
 __MATHDECL_PRIV (int,isnan,, (_Mdouble_ __value), (__const__))
+__END_NAMESPACE_C99
+#endif
 
+#if defined __USE_MISC || defined __USE_XOPEN
 # ifdef __DO_XSI_MATH__
 /* Bessel functions.  */
 __MATHCALL (j0,, (_Mdouble_))