Use dependencies in the INSTALL_PREFIX by default

[lpetre-ulb]

Title says it all. If the required dependencies exist the the INSTALL_PREFIX, they should be found and use automatically by default. This applies to xhal, reedmuller-c and possibly other GEM libraries.

[jsturdy]

I would oppose this on the grounds that INSTALL_PREFIX is designed and intended to be "package local", i.e., not telling the build system about other packages (which is not to say that other packages may live in there)

[lpetre-ulb]

I agree that this specific proposal may not be the best one, for multiple reasons. However, I'm convinced that easy installation in local installation must not be overlooked. It is essential for the easiness of development.

Another solution would be to have a ADDITIONAL_SEARCH_DIR which would implement the previously described functionality.