scons: Fix libicu version mismatch between linking-time and run-time

We have to use self-compiled ICU and not the one from the system to ensure
the same C++ ABI (libstdc++ from GNU) to be used by all the libraries
the Agent is linked against.

This doesn't fix the dependency with vnswad binary problem. The libicu still
has to be build explicitly before the Agent. This is currently handled by Devstack.