math: Fix errors in C++ mode on glibc systems with clang.
* lib/math.in.h (GNULIB_NAMESPACE_LACKS_ISFINITE,
GNULIB_NAMESPACE_LACKS_ISINF, GNULIB_NAMESPACE_LACKS_ISNAN,
GNULIB_NAMESPACE_LACKS_SIGNBIT): New macros.
* tests/test-math-c++.cc (isfinite): Skip test if
GNULIB_NAMESPACE_LACKS_ISFINITE is 1.
(isinf): Skip test if GNULIB_NAMESPACE_LACKS_ISINF is 1.
(isnan): Skip test if GNULIB_NAMESPACE_LACKS_ISNAN is 1.
(signbit): Skip test if GNULIB_NAMESPACE_LACKS_SIGNBIT is 1.