debug mode performance patch

[Paolo Carlini]

... first blush looks pretty good to me. But I see now an unconditional
use of <mutex>, which isn't always available, see:

...

#if defined(_GLIBCXX_HAS_GTHREADS) && defined(_GLIBCXX_USE_C99_STDINT_TR1)

...

Thus if we commit this kind of change to debug.cc the bootstrap breaks
for some targets. Please figure out a solution not relying
unconditionally on <mutex>.

Paolo.