configure: require C++11
configure: require C++11

Rationale: the adaptive plugin requires C++11; it won´t build on a C++98
compiler or C++11 compiler in C++98 mode.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc