Fix cross-compilation

[Palladinium]

The current build.rs uses cfg!() statements to check the build platform, which fails when cross-compiling (e.g. by using cross), because in build scripts those checks return information about the host system rather than the target system.

This PR changes the build script to use the environment variables provided by cargo to hopefully fix cross-compilation without affecting native compilation.

I've only been able to test Windows and Linux, as I don't have access to a Mac OS system.