Qt: Append -std=c++11 if building against Qt 5.7

• m4/qt.m4: Append -std=c++11 to CFLAGS if building against Qt 5.7

Qt 5.7 enables C++11 for Qt modules, and any app relying on it require to be
compiled with at least this standard.

This patch adds detection for Qt 5.7 and make sure -std=c++11 is passed if
building against Qt 5.7 or higher.