How do I use signtool in post build event for Visual Studio 2013 without hard coding path?

I've created a post build event to do code signing of the application after a successful build with the following post build script.

copy $(TargetPath) $(TargetDir)SignedApp.exe
signtool sign /t http://timestamp.verisign.com/scripts/timestamp.dll /a $(TargetDir)SignedApp.exe

I get the error 'signtool' is not recognized as an internal or external command. So it seems the path used for the build event doesn't point to the signtool utility. When I run the VS2013 x86 Native Tools Command Prompt I can run signtool as it includes a path which points to:

C:\Program Files (x86)\Windows Kits\8.1\bin\x86

I could hard-code this path into my build event

"C:\Program Files (x86)\Windows Kits\8.1\bin\x86\signtool" sign /t http://timestamp.verisign.com/scripts/timestamp.dll /a $(TargetDir)SignedApp.exe

However that seems non-portable. How do I get the same path defined for the Native Command Prompt to be used by my post build event without hard coding it? I've looked at the list of macros but haven't found any that would be useful.

The solution I decided on was:

REM If SIGNTOOL environment variable is not set then try setting it to a known location
if "%SIGNTOOL%"=="" set SIGNTOOL=%ProgramFiles(x86)%\Windows Kits\8.1\bin\x86\signtool.exe
REM Check to see if the signtool utility is missing
if exist "%SIGNTOOL%" goto OK1
    REM Give error that SIGNTOOL environment variable needs to be set
    echo "Must set environment variable SIGNTOOL to full path for signtool.exe code signing utility"
    echo Location is of the form "C:\Program Files (x86)\Windows Kits\8.1\x86\bin\signtool.exe"
    exit -1
:OK1
echo Copying $(TargetFileName) to $(TargetDir)SignedApp.exe
copy $(TargetPath) $(TargetDir)SignedApp.exe
"%SIGNTOOL%" sign /t http://timestamp.verisign.com/scripts/timestamp.dll /a $(TargetDir)SignedApp.exe

This was a variation on @Dennis Kuypers suggestion #4. The developer must set environment variable SIGNTOOL to the correct location. If they fail to do so then one known possible location is attempted. If that fails then error is reported instructing them to set SIGNTOOL env var appropriately.

I did discover there is an environment variable WindowsSdkDir

WindowsSdkDir=C:\Program Files (x86)\Windows Kits\8.1\

But again, this was set only when running the Native Command Prompt and thus was not defined when running the post build event script.