configure: Test for response file support in a way that works with busybox-w32

Don't use /dev/null as test input file for the test for response
file support.

Busybox on Windows does provide POSIX utilities to the point of
being able to build and test ffmpeg, but it doesn't provide full
POSIX emulation like msys2 and cygwin do.

Busybox utils do handle paths like /dev/null themselves. But when
busybox invokes a native Windows executable (like the "$ar" tool
in this test), it passes such a path as-is. Msys2 on the other hand
rewrites unixy paths in command line parameters - even when prefixed;
e.g. "@/dev/null" gets rewritten into "@nul".

Instead of testing with a /dev/null input, test with a real temp
file instead.

configure

@@ -5144,6 +5144,7 @@ tmpfile TMPO   .o
 tmpfile TMPS   .S
 tmpfile TMPSH  .sh
 tmpfile TMPV   .ver
+tmpfile TMPRSP .rsp
 
 unset -f mktemp
 
@@ -8268,7 +8269,8 @@ esac
 
 if [ "$response_files" != "no" ]; then
     ar_out=${FFTMPDIR}/test$LIBSUF
-    respfile="@/dev/null"
+    echo > "$TMPRSP"
+    respfile="@$TMPRSP"
     out_arg="$(echo $ar_o | sed "s;\$@;$ar_out;g")"
     if test_cmd $ar $arflags $out_arg $respfile; then
         response_files="yes"