Improve the way the --c++11 option is handled

When --c++11 is specified set CXXFLAGS instead of CCFLAGS
(since it only applies to C++).
Also, the details of which flag to set depend on the compiler
version (at least for gcc).
Fix a doxygen warning by getting rid of an obsolete option.
Implement review suggestions

Switch back from subprocess to context.TryAction(SCons.Script.Action
Fix C++11 test for gcc so 4.6 uses c++0x.
Merge branch 'tickets/DM-1209'