configure: test for C++11 support more often
configure: test for C++11 support more often

Whenever imwri will be compiled, to be exact. Previously, it would only
test for C++11 support if imwri was explicitly enabled using its own
switch, not when using --enable-plugins.

vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth