Add an exe extension variable and use it in the mingw build.
Add an exe extension variable and use it in the mingw build.

xiph/opusfile,gcp/opusfile,xiph/opusfile,gcp/opusfile,gcp/opusfile,SmartWalkieOrg/opusfile,SmartWalkieOrg/opusfile,youfearm3/opusfile,xiph/opusfile,SmartWalkieOrg/opusfile,youfearm3/opusfile