Fix compiler & linker flags usage

I tried to link openvpn statically against runtime using openvpn-build and faced a few issues listed below (fixes applied).

* -Xcompiler is not accepted by neither GCC or MinGW-w64 (at least not anymore).
* LDFLAGS (as well as CFLAGS) is cleared in the "build" script rendering DO_REALLY_STATIC much less useful.
* Shouldn't DO_REALLY_STATIC also mean static runtime? I'm not sure.