logl: Document musl libc bug

logl: Document musl libc bug

[Bruno Haible]

logl lacks precision on musl libc 1.2.2/arm64, musl libc 1.2.2/s390x.
Fortunately the configure test meant for NetBSD already catches it.


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

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

diff --git a/doc/posix-functions/logl.texi b/doc/posix-functions/logl.texi
index 1e7918b..e0d04b5 100644
--- a/doc/posix-functions/logl.texi
+++ b/doc/posix-functions/logl.texi
@@ -20,7 +20,7 @@ glibc 2.7 on Linux/SPARC64.
 @item
 This function produces results which are accurate to only 16 digits on some
 platforms:
-NetBSD 9.0.
+musl libc 1.2.2/arm64, musl libc 1.2.2/s390x, NetBSD 9.0.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/logl.m4 b/m4/logl.m4
index 128170a..7dcdac1 100644
--- a/m4/logl.m4
+++ b/m4/logl.m4
@@ -1,4 +1,4 @@
-# logl.m4 serial 14
+# logl.m4 serial 15
 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,
@@ -166,7 +166,7 @@ int main (int argc, char *argv[])
     if (!(gy + gy == gy))
       result |= 1;
   }
-  /* This test fails on NetBSD 9.0.  */
+  /* This test fails on musl 1.2.2/arm64, musl 1.2.2/s390x, NetBSD 9.0.  */
   {
     const long double TWO_LDBL_MANT_DIG = /* 2^LDBL_MANT_DIG */
       (long double) (1U << ((LDBL_MANT_DIG - 1) / 5))
@@ -188,8 +188,8 @@ int main (int argc, char *argv[])
         [case "$host_os" in
                           # Guess yes on glibc systems.
            *-gnu* | gnu*) gl_cv_func_logl_works="guessing yes" ;;
-                          # Guess yes on musl systems.
-           *-musl*)       gl_cv_func_logl_works="guessing yes" ;;
+                          # Guess no on musl systems.
+           *-musl*)       gl_cv_func_logl_works="guessing no" ;;
                           # Guess yes on native Windows.
            mingw*)        gl_cv_func_logl_works="guessing yes" ;;
                           # If we don't know, obey --enable-cross-guesses.