build: Reviving support for precompiled headers

This commit also includes an update of ./waf, which has several
improvements.  In particular, clang++ is now default compiler on OSX
platform.

This commit also includes reorganization of tests. All unit tests are
now under tests/unit-tests and integrated tests are under
tests/integrated.  This change allows small compilation optimization,
partially related to precompiled headers.

Change-Id: I4c171c04d18e9cb83e461264a35b9ed85ea4d50e