Support building with cross-compilers (eg: building Windows binaries on Linux)

[micolous]

Building on non-Windows platforms while also being able to test on Windows needs a Windows VM. ProxSpace is quite big, and then the overhead of Windows itself adds more.

Right now the build system in PM3 presumes that the host system is the target system.

It should be possible to cross-compile Proxmark3 for another system, eg: I can use Linux mingw32/64 packages to build Windows executables, and test them in burner Windows VMs, rather than needing to keep an environment operational.

In Fedora there are some helper scripts for this that give commands like mingw32-make, mingw32-autoconf, mingw32-cmake.

It may be easier if Proxmark adopted a build system that handled cross-compiling natively. I've already got working CMake scripts which handle cross-compiling for Android -- I'd be happy to get that into a state it can be upstreamed (as it has several Android-specific tweaks), but it means you'd need CMake to compile the project.