How can I read an environment variable in Fortran?

I'm trying to read an environment variable set by Matlab's setenv() function inside a Fortran .mex. It sounds simple enough... My mex function in Fortran looks essentially like:

        SUBROUTINE MEXFUNCTION(NLHS, PLHS, NRHS, PRHS)
        INTEGER NLHS, NRHS
        mwPointer PLHS(*), PRHS(*)
        mwSize maxbuff
        PARAMETER(maxbuff = 512)
        CHARACTER(maxbuff) input_buf, output_buf
  C     Get the string contents (dereference the input pointer).
        status = mxGetString(prhs(1), input_buf, maxbuf)
        IF (status .NE. 0) THEN 
           call mexErrMsgIdAndTxt ('MATLAB:revord:readError',
       +                           'Error reading string.')
        ENDIF 
  C     Initialize outbuf_buf to blanks. This is necessary on some compilers.
        output_buf = ' '
  C     Call the computational subroutine.
        CALL GETENV(input_buf,output_buf)
  C     Set output_buf to MATLAB mexFunction output.
        plhs(1) = mxCreateString(output_buf)      
        IF (plhs(1) .EQ. 0) THEN
            CALL mexErrMsgIdAndTxt ('FORTRAN:getenvmf:pushevalue',
       +        'Error pushing evalue to Matlab.')
        ENDIF
  C
        RETURN
        END

I compile and run it as:

>> mex getenvf.f -f mexopts64.bat
>>getenvf('OS')
ans =
Windows_NT  <-- The pre-existing DOS environment is fine.
>> setenv('Foo','bar')
>> getenv('Foo')
ans =
bar  <-- Matlab's getenv() works fine.
>> getenvf('Foo')
ans =
     ''  <-- The MATLAB-added environment variable is not loaded to the .mex's workspace.

Neither 32 nor 64 bit R2013b works. Any ideas as to where I went wrong?

getenvf.zip includes the following files:

getenvf.f <-- Fortran file containing a mexFunction that calls Fortran's getenv() subroutine and returns the value.

mexopts32.bat <--Option 9 modified for Intel Visual Fortran 13.0 SP1 (with Microsoft Visual C++ 2012 linker)

mexopts64.bat <--Option 9 modified for Intel Visual Fortran 13.0 SP1 (with Microsoft Visual C++ 2012 linker).

getenvf.mexw32 <-- my resulting .mexw32.

getenvf.mexw64 <-- my resulting .mexw64.

[Bob Clark]

New information: Compiling in R2013b generates a mexw32 that works in all versions of Matlab before R2013b. Compiling my sample code in R2011b generates a getenvf.mexw32 that works in all versions. Its something to do with R2013b.