Made MinGW statically link libgcc to patch run-time renderer error

Compiling on Windows using `make` through MinGW with GCC v.4.8.1-4 and `USE_RENDERER_DLOPEN=1` results in *libgcc* becoming dynamically linked in the renderer DLLs. This again results in the following run-time error when the engine loads the renderer:

    The program can't start because libgcc_s_dw2-1.dll is missing from your
    computer. Try reinstalling the program to fix this problem.

This pull request ensures that libgcc is statically linked during the build. It is not an issue when cross-compiling.