Hi,

isnan is a C99 feature. In the __USE_ISOC99-flag is used to define it.
And the macro expand to private functions __isnanf and __isnan
But in bits/mathcalls.h the flag isn't and the namespace isn't used either.

So we can get the error __isnanf or __isnan implicit declaration.

In bits/mathcalls.h we differ from glibc anyway and denis comment out
the PRIVCALL to __isnan anyway.

The attached patch 

1. removes the PRIVCALL to __isnan entirely, because
   noboy should call it directly from his code
2. sets the _USE_ISOC99-flag and c++ namespace for the isnan PRIVCALL

3. seperates out the BESSEL function, because I didnt looked up if they
   STDC99 too