Try using Pre-Compiled headers. Saves you a lot of time .
When you declare a variable in Main.cpp, it isn't included in any other files, and it's generally bad practice to include "Main.cpp" in another .h or .cpp file. What you could do is make a new file called "Global.h" and decalre all your Global variables in that file. That way, all you have to do is include "Global.h" in all of your source files.