Add stdin=subprocess.PIPE to prevent fail on win

[apetri]

I modified makePDF.py adding a stdin=subprocess.PIPE argument to external_command(); without this building fails on Windows 7

[r-stein]

Not sure if this leads to new problems. This build version worked for me on windows 7.

[apetri]

For some reason it still doesn’t work on mine, subprocess.Popen fails without the stdin argument. It could be my win 7 build