compile.sh library cache: autoconf, cmake etc need to be run again if install prefix changed

[dktapps]

One use case for caching is to reuse compiled dependencies for binaries installed in different locations.

Currently, the library caching only works if the prefix did not change.

[dktapps]

It turns out that changing prefix path causes most libraries to do a full recompile, rendering the cache mostly useless. We can track the target install path and avoid reconfiguring if it's unchanged, but it's less than ideal.