Friendlier windows build

only three env variables should already be defined before invoking eval `./otp_build env_win64`, the script takes care setting the rest of the variables
  MSDEV_PATH  (eg. C:\Program Files (x86)\Microsoft Visual Studio 10.0)
  PLATFORMSDK_PATH (eg. C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A)
  JDK_HOME (eg. /cygdrive/c/Program Files/Java/jdk1.7.0_03)


I'm using 2.69 myself, but I think 2.59 is a deliberate minimum version choice made by the OTP team. More importantly, I was curious if this change is required for a friendlier Windows build. As that's not the case, it's better to extract this change into a separate commit (explaining the motivation in its commit message) to make it clearer and less surprising.