isnan: Fix compilation error in C++ mode on OpenBSD 7.5-beta.
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=431d6a7615245e6b32d95b4b27aab5d3af65ad2b

Index: gettext-tools/gnulib-lib/math.in.h
--- gettext-tools/gnulib-lib/math.in.h.orig
+++ gettext-tools/gnulib-lib/math.in.h
@@ -2668,7 +2668,7 @@ _GL_MATH_CXX_REAL_FLOATING_DECL_1 (isnan)
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan, rpl_isnan, bool)
 #    define isnan rpl_isnan
 #    define GNULIB_NAMESPACE_LACKS_ISNAN 1
-#   elif (((defined __APPLE__ && defined __MACH__) || defined __FreeBSD__) && __clang_major__ >= 14)
+#   elif __clang_major__ >= 14
   /* Neither of the two possible _GL_MATH_CXX_REAL_FLOATING_DECL_2 invocations
      works.  Inline functions are already present in /usr/include/c++/v1/math.h,
      which comes from LLVM.  */