this is the same as provided in godot docs (GDNative C++ example) except that this one is modified to support compiling in linux for windows.
Example command:
scons platform=linux -j4 xwin=yes target=release target_name=test.dll
the parameter xwin=yes means that it will x-compile to windows.
target_name=test.dll the output file (needs testing maybe using .so will work on windows too).