Allow compilation with GNU Make 4.0.
Patch from <http://lists.gnu.org/archive/html/qemu-devel/2013-10/msg02758.html>.
Anaylsis at <http://lists.gnu.org/archive/html/bug-make/2013-10/msg00151.html>.

--- a/configure
+++ b/configure
@@ -308,6 +308,9 @@ query_pkg_config() {
 pkg_config=query_pkg_config
 sdl_config="${SDL_CONFIG-${cross_prefix}sdl-config}"
 
+# If the user hasn't specified ARFLAGS, default to 'rv', just as make does.
+ARFLAGS="${ARFLAGS-rv}"
+
 # default flags for all hosts
 QEMU_CFLAGS="-fno-strict-aliasing $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
@@ -3664,6 +3667,7 @@ echo "C compiler        $cc"
 echo "Host C compiler   $host_cc"
 echo "C++ compiler      $cxx"
 echo "Objective-C compiler $objcc"
+echo "ARFLAGS           $ARFLAGS"
 echo "CFLAGS            $CFLAGS"
 echo "QEMU_CFLAGS       $QEMU_CFLAGS"
 echo "LDFLAGS           $LDFLAGS"
@@ -4239,6 +4243,7 @@ echo "HOST_CC=$host_cc" >> $config_host_mak
 echo "CXX=$cxx" >> $config_host_mak
 echo "OBJCC=$objcc" >> $config_host_mak
 echo "AR=$ar" >> $config_host_mak
+echo "ARFLAGS=$ARFLAGS" >> $config_host_mak
 echo "AS=$as" >> $config_host_mak
 echo "CPP=$cpp" >> $config_host_mak
 echo "OBJCOPY=$objcopy" >> $config_host_mak
