build: detect the right `ar` tool

aclocal.m4

Index: aclocal.m4
diff --git a/aclocal.m4 b/aclocal.m4
index e657517b01ac0c7d67e7fc0393ddd74a247ad0c8..10010c22931918a9e0dfd835e9b55ce887452383 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -56,6 +56,66 @@ m4_ifndef([AC_AUTOCONF_VERSION],
   [m4_copy([m4_PACKAGE_VERSION], [AC_AUTOCONF_VERSION])])dnl
 _AM_AUTOCONF_VERSION(m4_defn([AC_AUTOCONF_VERSION]))])
 
+# Copyright (C) 2011-2014 Free Software Foundation, Inc.
+#
+# This file is free software; the Free Software Foundation
+# gives unlimited permission to copy and/or distribute it,
+# with or without modifications, as long as this notice is preserved.
+
+# AM_PROG_AR([ACT-IF-FAIL])
+# -------------------------
+# Try to determine the archiver interface, and trigger the ar-lib wrapper
+# if it is needed.  If the detection of archiver interface fails, run
+# ACT-IF-FAIL (default is to abort configure with a proper error message).
+AC_DEFUN([AM_PROG_AR],
+[AC_BEFORE([$0], [LT_INIT])dnl
+AC_BEFORE([$0], [AC_PROG_LIBTOOL])dnl
+AC_REQUIRE([AM_AUX_DIR_EXPAND])dnl
+AC_REQUIRE_AUX_FILE([ar-lib])dnl
+AC_CHECK_TOOLS([AR], [ar lib "link -lib"], [false])
+: ${AR=ar}
+
+AC_CACHE_CHECK([the archiver ($AR) interface], [am_cv_ar_interface],
+  [AC_LANG_PUSH([C])
+   am_cv_ar_interface=ar
+   AC_COMPILE_IFELSE([AC_LANG_SOURCE([[int some_variable = 0;]])],
+     [am_ar_try='$AR cru libconftest.a conftest.$ac_objext >&AS_MESSAGE_LOG_FD'
+      AC_TRY_EVAL([am_ar_try])
+      if test "$ac_status" -eq 0; then
+        am_cv_ar_interface=ar
+      else
+        am_ar_try='$AR -NOLOGO -OUT:conftest.lib conftest.$ac_objext >&AS_MESSAGE_LOG_FD'
+        AC_TRY_EVAL([am_ar_try])
+        if test "$ac_status" -eq 0; then
+          am_cv_ar_interface=lib
+        else
+          am_cv_ar_interface=unknown
+        fi
+      fi
+      rm -f conftest.lib libconftest.a
+     ])
+   AC_LANG_POP([C])])
+
+case $am_cv_ar_interface in
+ar)
+  ;;
+lib)
+  # Microsoft lib, so override with the ar-lib wrapper script.
+  # FIXME: It is wrong to rewrite AR.
+  # But if we don't then we get into trouble of one sort or another.
+  # A longer-term fix would be to have automake use am__AR in this case,
+  # and then we could set am__AR="$am_aux_dir/ar-lib \$(AR)" or something
+  # similar.
+  AR="$am_aux_dir/ar-lib $AR"
+  ;;
+unknown)
+  m4_default([$1],
+             [AC_MSG_ERROR([could not determine $AR interface])])
+  ;;
+esac
+AC_SUBST([AR])dnl
+])
+
 # Figure out how to run the assembler.                      -*- Autoconf -*-
 
 # Copyright (C) 2001-2014 Free Software Foundation, Inc.