Merge: Java FFI global compilation fix

Fixes:
* Undefined references on unused callbacks with nitg-g
* Duplicated functions on callbacks present in 2 modules

Pull-Request: #879
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc src/c_tools.nit
Simple merge

diff --cc src/ffi/cpp.nit
Simple merge

diff --cc src/ffi/java.nit
Simple merge