Explicitly disable C++11 during the build.

Prepare for Qt 5.6, which will pass -std=gnu++11 by default when the compiler
supports it, and the build currently does not work:

../TelegramQML-0.9.2/telegramqml.cpp:4794:10: error: case value evaluates to 2573335900, which cannot be narrowed to type 'int' [-Wc++11-narrowing]

case Update::typeUpdateReadHistoryInbox:

Note that Qt 5.7 will start requiring C++11 support, so this will need to be
fixed properly in the future.

PR: 211916