configure: Print a banner comment at the top of config.log

Print a banner comment at the top of config.log identifying
when configure was run and the arguments used. This is occasionally
useful for debugging purposes.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -20,6 +20,11 @@ TMPE="${TMPDIR1}/qemu-conf-${RANDOM}-$$-${RANDOM}.exe"
 trap "rm -f $TMPC $TMPO $TMPE" EXIT INT QUIT TERM
 rm -f config.log
 
+# Print a helpful header at the top of config.log
+echo "# QEMU configure log $(date)" >> config.log
+echo "# produced by $0 $*" >> config.log
+echo "#" >> config.log
+
 compile_object() {
   echo $cc $QEMU_CFLAGS -c -o $TMPO $TMPC >> config.log
   $cc $QEMU_CFLAGS -c -o $TMPO $TMPC >> config.log 2>&1