Warning from chrono that could helpfully be suppressed

[Paul A. Bristow]

Boost.Chrono is emitting warnings with GCC (5.1.0) that cannot be suppressed even using the following cxxflags in the user jamfile

      <toolset>gcc:<cxxflags>-Wno-deprecated-declarations
      <toolset>gcc:<cxxflags>-Wno-long-long
      <toolset>gcc:<cxxflags>-ftrack-macro-expansion=0 # Suppress note: in expansion of macro

and they are repeated on every compilation which is quite annoying when reading the log file.

For example:

gcc.compile.c++ ..\..\..\bin.v2\libs\timer\build\gcc-mingw-5.1.0\debug\link-static\auto_timers_construction.o
In file included from C:/Program Files/mingw-w64/x86_64-5.1.0-win32-seh-rt_v4-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/5.1.0/include/stdint.h:9:0,
                 from ..\..\../boost/cstdint.hpp:60,
                 from ..\..\../boost/ratio/config.hpp:13,
                 from ..\..\../boost/ratio/ratio.hpp:35,
                 from ..\..\../boost/chrono/duration.hpp:41,
                 from ..\..\../boost/chrono/chrono.hpp:11,
                 from ..\..\../boost/timer/timer.hpp:14,
                 from ..\..\..\libs\timer\src\auto_timers_construction.cpp:23:
..\..\../boost/chrono/system_clocks.hpp:77:111: warning: use of C++11 long long integer constant [-Wlong-long]
 # define BOOST_SYSTEM_CLOCK_DURATION

boost::chrono::duration<boost::int_least64_t, ratio<BOOST_RATIO_INTMAX_C(1), BOOST_RATIO_INTMAX_C(10000000)> >
                                                                                                               ^
..\..\../boost/chrono/system_clocks.hpp:77:90: note: in expansion of macro 'BOOST_RATIO_INTMAX_C'
 # define BOOST_SYSTEM_CLOCK_DURATION boost::chrono::duration<boost::int_least64_t, ratio<BOOST_RATIO_INTMAX_C(1), BOOST_RATIO_INTMAX_C(10000000)> >

and more...

It would be nice if the user would not see these (spurious?) warnings.