boost: context needs C++11

boost: context needs C++11

Without std=c++11, mutex is not always correctly detected with gcc 4.7,
4.8 or even 5.x. As a result, boost_thread is wrongly selected by
boost_context.

mutex is available on gcc 4.7.x, the issue is that boost does not
correctly detect it because -std=c++11 was missing.
Keep thread select for gcc version lower or equal to 4.6. mutex is
available since gcc 4.4 (with std=c++0x) but common buildroot practice
for C++11 packages is to have a dependency on gcc 4.7, 4.8 or 4.9
depending on the requested C++11 features.

Fixes:
 - http://autobuild.buildroot.net/results/fb046c04fe18bec973d120e4ab33971f32ba5769
 - http://autobuild.buildroot.net/results/99f1a255b78a973faeb8bf3b94d78efc54426a8b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>