Merge: Docker, portability and crosscompiling tweaks

* Add the missing SDL2 package (since #2376) to the full DockerFile.
* Fix a warning raised by the C compiler when called by niti on 32 bits platforms.
* Clean up Makefile generation and enable using environment variables to override the default C compiler, C++ compiler, CFLAGS and whether or not to use libunwind. Making it possible to change the compiler for cross compilation and to skip linking with libunwind to create binaries compatible with more GNU/Linux systems.

Pull-Request: #2390
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>

diff --cc src/compiler/abstract_compiler.nit
Simple merge