Installing a framework anywhere but a reasonably standard location doesn't work. You should try to build with "--enable-framework=$SOMEPREFIX/Library/Frameworks". Applications then get installed in "$SOMEPREFIX/Applications" and command-line tools in "$SOMEPREFIX/bin". When SOMEPREFIX is /Library or /System/Library items get installed in the usual location (but don't install into /System/Library unless you are Apple).

Supporting an arbitrary framework prefix would require a clear definition on what to do with Application bundles and command-line tools and I don't have enough of a need for that to start thinking about that.