Build: pass --build to configure when cross-compiling.

This is the right thing to do, and also happens to fix building
with MinGW-w64 in a non-Linux environment such as e.g. Cygwin.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6104 b64f7644-9d1e-0410-96f1-a4d463321fa5