snippet/c++defs: fix real-floating arg functions in C++ mode
Also, define isfinite, isinf, isnan, signbit in the gnulib
namespace instead of in the global namespace.
* build-aux/snippet/c++defs.h (_GL_BEGIN_NAMESPACE)
(_GL_END_NAMESPACE): New.
* lib/math.in.h (_GL_MATH_CXX_REAL_FLOATING_DECL_2): Use them.
(isfinite, isinf, isnan, signbit) [__cplusplus &&
GNULIB_NAMESPACE]: Define them in the GNULIB_NAMESPACE namespace
instead of in the global namespace.
* tests/test-math-c++.cc: Check that the isfinite, isinf, isnan,
signbit overloads exist in the GNULIB_NAMESPACE namespace, instead
of in the global namespace.