Extend jam to track the contents of variables during the build.

[mmadia]

As mentioned in #7807, jam does not account for variable changes that affected compiled objects, such as -sHAIKU_DISTRO_COMPATIBILITY=default

[anevilyak]

(In #7807) No, I just picked the wrong drop-down option.

[waddlesplash]

I don't really know what it could do here besides discarding everything; it sets this as a global define so theoretically all files are affected...