Starting from C++11, several math functions, like isinf, moved into the std
namespace. Since cmath undefines those functions before redefining them inside
the namespace, and glibc 2.23 defines the C variants as macros, the C variants
in global namespace are not accessible any longer.

v2: Move the fix outside of Nouveau, as suggested by Jose Fonseca, since anyone
    might need it when GCC switches to C++14 by default with GCC 6.0.

v3:
*   Put the code directly inside c99_math.h rather than creating a new header
    file, as asked by Jose Fonseca;
*   Guard the code behind glibc version checks, as only glibc > =2.23 defines
    isinf & co. as functions, as suggested by Jose Fonseca.

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Jose Fonseca <jfonseca@vmware.com>