You can try adding
QMAKE_CXXFLAGS += -std=c++0x
to your .pro file.
However, this should not be used in Qt 5 for enabling specific c++ standard. Instead, c++11 or c++14 in CONFIG variable to do that. It will enable GNU extensions (-std=gnu++11), but if that is unwanted, also add strict_c++ if you want to disable those. For example, this should pass -std=c++11 to the compiler:
CONFIG += c++11 strict_c++