isfinite, isinf, isnan, signbit: Fix error in C++ mode on mingw.
Reported by Martin Storsjö <martin@martin.st> in
<https://lists.gnu.org/archive/html/bug-gnulib/2019-08/msg00075.html>.
* lib/math.in.h (_GL_MATH_CXX_REAL_FLOATING_DECL_2): Add more arguments.
(isfinite, isinf, isnan, signbit): On platforms that use C++ include
files from GCC 6 or newer, use an override through '#define', because
the inline definitions in the platform's <cmath> cannot be overridden
in another way.