Use arbitrary cross-compilation flags instead of inheriting the system ones

Turns out our system hardening flags could introduce breakage in some software running under Wine. It is not clear which ones exactly cause problems, but chances are different software could be hit by different ones.

This MR is a fixup for !6 (merged). It fixes potential problems by using arbitrary cross-compilation flags instead of inheriting the system ones. Most of the useful job is leveraged by that basic optimization flags anyway.

Fixes #13 (closed).

P.S. @felixonmars, this fix is not really urgent as the problem should be very rare, but better to be merged asap.