Some parts of boost need at least C++11, activate -std=gnu++14
when possible and otherwise use -std=gnu++11 instated of using the
default which is -std=gnu++98.

This fixes the build for me when I build everything expect
boost-coroutine2 with gcc 5.4.0 on LEDE.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>