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
diff --git a/waf b/waf
index 78a44f3..7828844 100755
--- a/waf
+++ b/waf
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