bits/mathcalls.h causes declarations in global namespace in C++

[Carlo Wood]

Compiling a C++ program that only included standard headers should never
define or declare anything outside of namespace std.

However, try to compile this:

#include <cmath>
int y0;

>g++ junk.cc
junk.cc:3:5: error: ‘int y0’ redeclared as different kind of symbol
 int y0;
     ^
In file included from /usr/include/features.h:374:0,
                 from /usr/include/x86_64-linux-gnu/c++/4.8/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/4.8/bits/c++config.h:426,
                 from /usr/include/c++/4.8/cmath:41,
                 from junk.cc:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:240:1: error: previous declaration of ‘double y0(double)’
 __MATHCALL (y0,, (_Mdouble_));
 ^

The file in question defines a lot (which is supposedly on purpose for the C language with GNU extension),
which is shouldn't do that when compiling a C++ program.