Better Scons file for GDNative C++ Documentation

[zeroxoneafour]

In this page, the Scons file is very limited in capability. It cannot compile with custom architectures or MinGW. Here is a better Scons file, tested by me with MinGW on Windows.
Inspired by this issue.

SConstruct.zip

[ryanpeach]

I also feel like the one provided should be able to auto detect platforms.

[Calinou]

@ryanpeach The SCons file from the documentation should probably be removed at this point. It would be better moved to the godot-cpp repository.

[philomelus]

I had to add the following

env.Tool('mingw')

below

if env['use_mingw']:

to the above SConstruct and it worked. Before adding the env.Tool line, it would generate a mixture of msvc and mingw commands (e.g. 'g++ /Fosrc/example.o ... instead of g++ -o src/example.o ...).