fenv-exceptions-tracking-c99: Work around a NetBSD/x86_64 bug

[Bruno Haible]

On NetBSD/x86_64, I see test failures in soon-to-be-added unit tests.
The cause is that fetestexcept(), which of course is meant to be
side-effect-free, clears the exception trap bits.

The bug is evident from looking at the definition of fetestexcept()
http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libm/arch/x86_64/fenv.c?rev=1.10&content-type=text/x-cvsweb-markup&only_with_tag=MAIN
It uses an 'fnstenv' instruction without subsequent 'fldenv' or 'fldcw'.


2023-11-04  Bruno Haible  <bruno@clisp.org>

        fenv-exceptions-tracking-c99: Work around a NetBSD/x86_64 bug.
        * m4/fenv-exceptions-tracking.m4 (gl_FENV_EXCEPTIONS_TRACKING): On
        NetBSD/x86_64, set REPLACE_FETESTEXCEPT to 1.
        * doc/posix-functions/fetestexcept.texi: Document the NetBSD bug.

diff --git a/doc/posix-functions/fetestexcept.texi 
b/doc/posix-functions/fetestexcept.texi
index b8eba903be..7e9b8e3327 100644
--- a/doc/posix-functions/fetestexcept.texi
+++ b/doc/posix-functions/fetestexcept.texi
@@ -11,6 +11,10 @@
 @item
 This function is missing on some platforms:
 FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, IRIX 6.5, Solaris 
9, Cygwin 1.7.7, MSVC 9, Android 4.4.
+@item
+This function clears the floating-point exception trap bits on some platforms:
+@c It uses an 'fnstenv' instruction without subsequent 'fldenv' or 'fldcw'.
+NetBSD 9.3/x86_64.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/fenv-exceptions-tracking.m4 b/m4/fenv-exceptions-tracking.m4
index a7ffa80784..1aa378f971 100644
--- a/m4/fenv-exceptions-tracking.m4
+++ b/m4/fenv-exceptions-tracking.m4
@@ -1,4 +1,4 @@
-# fenv-exceptions-tracking.m4 serial 1
+# fenv-exceptions-tracking.m4 serial 2
 dnl Copyright (C) 2023 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -20,6 +20,8 @@ AC_DEFUN_ONCE([gl_FENV_EXCEPTIONS_TRACKING]
     dnl On musl libc, on those CPUs where fenv-except-tracking-raise.c
     dnl uses the "generic" approach, feraiseexcept does not trigger traps
     dnl because it merely manipulates flags in the control register.
+    dnl On NetBSD 9.3/x86_64, fetestexcept clears the exception trap bits,
+    dnl causing the fenv-environment tests to fail.
     case "$host" in
       arm*-*-linux*)
         AC_CACHE_CHECK([whether feraiseexcept works],
@@ -110,6 +112,12 @@ AC_DEFUN_ONCE([gl_FENV_EXCEPTIONS_TRACKING]
           *) REPLACE_FERAISEEXCEPT=1 ;;
         esac
         ;;
+      x86_64-*-netbsd*)
+        dnl On NetBSD 9.3/x86_64, fetestexcept is broken: it clears the
+        dnl floating-point exception trap bits (because it uses an
+        dnl 'fnstenv' instruction without subsequent 'fldenv' or 'fldcw').
+        REPLACE_FETESTEXCEPT=1
+        ;;
     esac
     if test $REPLACE_FECLEAREXCEPT = 1 \
        && test $REPLACE_FETESTEXCEPT = 1 \