configure: Fix -redo when configure line contained equal sign(s)

If the original configure arguments contained equal sign(s) like in
    configure ... -- -DQT_BUILD_SUBMODULES=qtbase;...

The following error message was printed:
    =qtbase was unexpected at this time.

The reason is that the conditional
    if "%rargs%" == ""
would expand to something like
    if ""-DQT_BUILD_SUBMODULES=qtsvg"" == ""
which is invalid, because the equal sign is used outside of a string
literal.

Use delayed expansion in the conditional to fix this.

Pick-to: 6.2 6.3
Change-Id: I8c40d799db2c5aae8cba190ed31804c71a605935
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

configure.bat

@@ -103,7 +103,7 @@ goto doneargs
 cd "%TOPQTDIR%"
 
 rem Write config.opt if we're not currently -redo'ing
-if "%rargs%" == "" (
+if "!rargs!" == "" (
     echo %*>config.opt.in
     cmake -DIN_FILE=config.opt.in -DOUT_FILE=config.opt -DIGNORE_ARGS=-top-level -P "%QTSRC%\cmake\QtWriteArgsFile.cmake"
 )