lock: Fix cross-compilation guesses

[Bruno Haible]

Now that it's clear that glibc bug
<https://sourceware.org/bugzilla/show_bug.cgi?id=13701> will never be fixed,
it's time to update the corresponding cross-compilation guess in gnulib.


2019-08-29  Bruno Haible  <address@hidden>

        lock: Fix cross-compilation guesses.
        * m4/pthread_rwlock_rdlock.m4 (gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER):
        Require AC_CANONICAL_HOST. When cross-compiling, guess no on most
        platforms.

diff --git a/m4/pthread_rwlock_rdlock.m4 b/m4/pthread_rwlock_rdlock.m4
index 3c1d645..85e8dba 100644
--- a/m4/pthread_rwlock_rdlock.m4
+++ b/m4/pthread_rwlock_rdlock.m4
@@ -1,4 +1,4 @@
-# pthread_rwlock_rdlock.m4 serial 2
+# pthread_rwlock_rdlock.m4 serial 3
 dnl Copyright (C) 2017-2019 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -29,11 +29,13 @@ dnl and SCHED_RR, see
 dnl 
http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html
 dnl but this test verifies the guarantee regardless of TPS and regardless of
 dnl scheduling policy.
-dnl Glibc currently does not provide this guarantee, see
+dnl Glibc does not provide this guarantee (and never will on Linux), see
 dnl https://sourceware.org/bugzilla/show_bug.cgi?id=13701
+dnl https://bugzilla.redhat.com/show_bug.cgi?id=1410052
 AC_DEFUN([gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER],
 [
   AC_REQUIRE([gl_THREADLIB_EARLY])
+  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
   AC_CACHE_CHECK([whether pthread_rwlock_rdlock prefers a writer to a reader],
     [gl_cv_pthread_rwlock_rdlock_prefer_writer],
     [save_LIBS="$LIBS"
@@ -153,7 +155,25 @@ main ()
 ]])],
        [gl_cv_pthread_rwlock_rdlock_prefer_writer=yes],
        [gl_cv_pthread_rwlock_rdlock_prefer_writer=no],
-       [gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing yes"])
+       [case "$host_os" in
+                         # Guess no on glibc systems.
+          *-gnu* | gnu*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
no" ;;
+                         # Guess no on musl systems.
+          *-musl*)       gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
no" ;;
+                         # Guess no on bionic systems.
+          *-android*)    gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
no" ;;
+                         # Guess yes on native Windows with the mingw-w64 
winpthreads library.
+                         # Guess no on native Windows with the gnulib 
windows-rwlock module.
+          mingw*)        if test "$gl_use_threads" = yes || test 
"$gl_use_threads" = posix; then
+                           gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
yes"
+                         else
+                           gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
no"
+                         fi
+                         ;;
+                         # If we don't know, assume the worst.
+          *)             gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing 
no" ;;
+         esac
+       ])
      LIBS="$save_LIBS"
     ])
   case "$gl_cv_pthread_rwlock_rdlock_prefer_writer" in