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)


it's not, autoconf 2.68 (on cygwin) works fine, if there is a known bug in other platforms that change can be dropped