Fix/Suppress all -Wall -Wextra -Weffc++ warnings on gcc 4.6.x - 4.8.x

https://code.google.com/p/rapidjson/issues/detail?id=65

There are a lot of warning "base class ‘class X’ has a non-virtual destructor" due to Weffc++. This is intentional to reduce virtual function table pointer overhead for Value (other classes may be arguable). So those warnings are suppressed by pragama.