PCbuild/build.bat: Add note about using msbuild response file. (#1551)

Using a response file will eliminate the headache associated with batch argument/quote processing.

For example I unsucessfully compiled python with visualcpptools when specifying VSInstallDir in the batch file (cannot find vcruntime.h)

```batch
build.bat -p x64 -e -M --no-tkinter  "/p:VCInstallDir=%VCInstallDir%"
```

but it build successfully when specifying it in a response file

msbuild.rsp:
```
/p:VCInstallDir=%VCInstallDir%
```

1 files changed, 6 insertions, 1 deletions

diff --git a/PCbuild/build.bat b/PCbuild/build.bat
index fce1e46..aac3f7f 100644
--- a/PCbuild/build.bat
+++ b/PCbuild/build.bat
@@ -10,7 +10,12 @@ echo.given.
 echo.

 echo.After the flags recognized by this script, up to 9 arguments to be passed

 echo.directly to MSBuild may be passed.  If the argument contains an '=', the

-echo.entire argument must be quoted (e.g. `%~nx0 "/p:PlatformToolset=v100"`)

+echo.entire argument must be quoted (e.g. `%~nx0 "/p:PlatformToolset=v100"`).

+echo.Alternatively you can put extra flags for MSBuild in a file named 

+echo.`msbuild.rsp` in the `PCbuild` directory, one flag per line. This file

+echo.will be picked automatically by MSBuild. Flags put in this file does not

+echo.need to be quoted. You can still use environment variables inside the 

+echo.response file.

 echo.

 echo.Available flags:

 echo.  -h  Display this help message