math C++ tests: Fix compilation error with clang on FreeBSD 13

[Bruno Haible]

On FreeBSD 13.0, I get this compilation error:

In file included from ../../gltests/test-math-c++2.cc:20:
/usr/include/c++/v1/cmath:325:7: error: no member named 'rpl_isnan' in the 
global namespace; did you mean 'gnulib::rpl_isnan'?
using ::isnan;
      ^~
../gllib/math.h:3104:43: note: 'gnulib::rpl_isnan' declared here
_GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan, rpl_isnan, bool)
                                          ^
1 error generated.
*** Error code 1

The fix is to revert the change from 2020-12-09, that was done for
FreeBSD 12.2, for versions of FreeBSD ≥ 13.


2022-09-02  Bruno Haible  <bruno@clisp.org>

        math C++ tests: Fix compilation error with clang on FreeBSD 13.
        * lib/math.in.h (isnan): For clang >= 11 on FreeBSD, declare 'isnan',
        not 'rpl_isnan'.

diff --git a/lib/math.in.h b/lib/math.in.h
index a74a95da3e..270e71f414 100644
--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -2591,7 +2591,7 @@ _GL_EXTERN_C int rpl_isnanl (long double x) 
_GL_ATTRIBUTE_CONST;
 #  if defined isnan || defined GNULIB_NAMESPACE
 _GL_MATH_CXX_REAL_FLOATING_DECL_1 (isnan)
 #   undef isnan
-#   if __GNUC__ >= 6 || (defined __clang__ && !((defined __APPLE__ && defined 
__MACH__ && __clang_major__ < 12) || (defined __FreeBSD__ && __clang_major__ < 
7) || defined __OpenBSD__ || (defined _WIN32 && !defined __CYGWIN__)))
+#   if __GNUC__ >= 6 || (defined __clang__ && !((defined __APPLE__ && defined 
__MACH__ && __clang_major__ < 12) || (defined __FreeBSD__ && (__clang_major__ < 
7 || __clang_major__ >= 11)) || defined __OpenBSD__ || (defined _WIN32 && 
!defined __CYGWIN__)))
   /* This platform's <cmath> possibly defines isnan through a set of inline
      functions.  */
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan, rpl_isnan, bool)