Work around m4 bracket stripping in MINGWSTRIP construction

Signed-off-by: Rene Engelhard <rene@debian.org> (on IRC).

configure.in

@@ -3866,7 +3866,7 @@ if test "z$enable_odk" = "z" -o "$enable_odk" != "no"; then
       if echo "$WITH_MINGW" | $EGREP -q "/"; then
          if ! test -x "`echo $WITH_MINGW | $SED -e s/[cg]++/strip/`"; then MINGWSTRIP=false; else MINGWSTRIP=$(basename $(echo $WITH_MINGW | $SED -e s/[cg]++/strip/)); fi
       else
-         AC_CHECK_TOOL(MINGWSTRIP, `echo $WITH_MINGW | $SED -e s/[cg]++/strip/`, false)
+         AC_CHECK_TOOL(MINGWSTRIP, `echo $WITH_MINGW | $SED -e s/[[[cg]]]++/strip/`, false)
       fi
       if test "$MINGWSTRIP" = "false"; then
          AC_MSG_ERROR(MinGW32 binutils needed. Install them.)