Change c++11 behaviour of net-im/libqtelegram-ae and net-im/telegramqml.

Instead of explicitely disabling c++11 mode as the code uses narrowing of
unsigned ints to ints, simply pass -no-c++11-narrowing if the compiler is clang.
This is done in preparation for Qt 5.7.1, which requires c++11 mode.

Approved by: rakuco (mentor)