This is a partial revert of db8df57a "fixes for
debug flags handling."  Removing this caused various nastinesses, at least:

* Some tinderbox started to emit bogus compiler warnings at -O2 vs. -O0, see
  cf9c715a "Avoid bogus warnings (GCC 4.4.6)"

* General DBG_UTIL vs. OSL_DEBUG_LEVEL confusion in the code, see
  729165eb "OSL_DEBUG_LEVEL > 0 -> defined
  DBG_UTIL" and a7c7942f "DBG_UTIL vs
  OSL_DEBUG_LEVEL confusion"

* --enable-dbgutil would no longer prevent -DNDEBUG (i.e., assert became
  ineffective in such builds)