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.
Merge pull request #17 from Semihalf/dev-icu

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