For one, assert.h is designed to be includeable multiple times with changing
NDEBUG settings, so it is not robust to include it early in sal/macros.h with
"correct" NDEBUG settings and potentially include it again later.  For another,
there is #ifndef NDEBUG code providing functionality used exclusively within
assert calls, which must be compiled with the same NDEBUG-setting as the
relevant #include <assert.h>.

Change-Id: I7b2f9c85f8e2155051274757c64162ed5a5e9d1b