How to cross-compile DLL with exported functions
Pragmas are specific to each compiler, your pragmas will work in Visual C++, but not in MingW. Instead you can use a .def file which are supported by Visual C++ and MinGW.
Here's what injector.def might look like in your case:
EXPORTS CheckEvenOdd = GetEvenOdd.dll.original.CheckEvenOdd PrintAMessage = GetEvenOdd.dll.original.PrintAMessageCompilation command:
$ i686-w64-mingw32-g++ -o GetEvenOdd.dll --shared injector.cpp injector.def