expm1l: Document musl libc bug

[Bruno Haible]

expm1l lacks precision on musl libc 1.2.2/arm64, musl libc 1.2.2/s390x.
Fortunately the configure test meant for Mac OS X and NetBSD already catches it.


2021-01-31  Bruno Haible  <bruno@clisp.org>

        expm1l: Document musl libc bug.
        * doc/posix-functions/expm1l.texi: Document musl libc bug.
        * m4/expm1l.m4 (gl_FUNC_EXPM1L): Update comment and cross
        compilation guess.

diff --git a/doc/posix-functions/expm1l.texi b/doc/posix-functions/expm1l.texi
index 4cd58a8..059d5fc 100644
--- a/doc/posix-functions/expm1l.texi
+++ b/doc/posix-functions/expm1l.texi
@@ -17,7 +17,7 @@ IRIX 6.5.
 @item
 This function produces results which are accurate to only 16 digits on some
 platforms:
-Mac OS X 10.5, NetBSD 8.0.
+musl libc 1.2.2/arm64, musl libc 1.2.2/s390x, Mac OS X 10.5, NetBSD 8.0.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/expm1l.m4 b/m4/expm1l.m4
index 8fe31b5..ec0d907 100644
--- a/m4/expm1l.m4
+++ b/m4/expm1l.m4
@@ -1,4 +1,4 @@
-# expm1l.m4 serial 8
+# expm1l.m4 serial 9
 dnl Copyright (C) 2010-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -123,7 +123,8 @@ int main (int argc, char *argv[])
 {
   long double (* volatile my_expm1l) (long double) = argc ? expm1l : dummy;
   int result = 0;
-  /* This test fails on Mac OS X 10.5, NetBSD 8.0.  */
+  /* This test fails on musl 1.2.2/arm64, musl 1.2.2/s390x, Mac OS X 10.5,
+     NetBSD 8.0.  */
   {
     const long double TWO_LDBL_MANT_DIG = /* 2^LDBL_MANT_DIG */
       (long double) (1U << ((LDBL_MANT_DIG - 1) / 5))
@@ -147,8 +148,8 @@ int main (int argc, char *argv[])
             [case "$host_os" in
                               # Guess yes on glibc systems.
                *-gnu* | gnu*) gl_cv_func_expm1l_works="guessing yes" ;;
-                              # Guess yes on musl systems.
-               *-musl*)       gl_cv_func_expm1l_works="guessing yes" ;;
+                              # Guess no on musl systems.
+               *-musl*)       gl_cv_func_expm1l_works="guessing no" ;;
                               # Guess yes on native Windows.
                mingw*)        gl_cv_func_expm1l_works="guessing yes" ;;
                               # If we don't know, obey --enable-cross-guesses.