Fix for unexpected mingw version

[danjujan]

This should fix compiling for x86_64 under ubuntu where mingw outputs the unexpected version 10-win32 leading to the following build error.

/bin/sh: 1: [: Illegal number: 10-win32
/bin/sh: 1: [: Illegal number: 10-win32

Needed for https://github.com/pbatard/efifs/pull/46