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 mingw, use an override through '#define', because the inline definitions in the platform's <cmath> cannot be overridden in another way.