Set default compiler warning flags
Set default compiler warning flags

Set the CMAKE_CXX_FLAGS default value for GCC and Clang compilers to
have -Wall and -Wextra, yet allow these values to be overriden in the
GUI

electronifie/SimpleAmqpClient,tangkingchun/SimpleAmqpClient,alanxz/SimpleAmqpClient,NETkiddy/SimpleAmqpClient,electronifie/SimpleAmqpClient,alanxz/SimpleAmqpClient