[SCons] Add support for custom build tools and platforms

[Faless]

Use with:

scons platform=os2 custom_tools=/path/to/tools

(assuming you have an os2.py inside /path/to/tools/)

E.g.:

# /path/to/tools/os2.py
def exists(env):
    return True


def generate(env):
    if env["arch"] not in ("x86_32", "ppc32"):
        print("Only x86_32 and ppc32 are supported on OS/2. Exiting.")
        env.Exit(1)

    env["CC"] = "echo"
    env["CXX"] = "echo"
    env["AR"] = "echo"
    env["LINK"] = "echo"
    env["RC"] = "echo"
    env["RANLIB"] = "echo"

[Faless]

Some notes (for future PRs maybe):

It might be nice to be able to override the allowed architectures (though it might end up being a feature creep)

It might be nice to add the possibility to override the default platforms with custom tools (Edit: Done via the patch mentioned below)

We might want to move (some or all) the "targets.py" back into the platform tools (maybe as an import?)
The main reason is that "targets" right now ends up making assumptions on build/link flags based on the default platforms which may be wrong when using custom tools. (EDIT: Proposed in #1392)

[dsnopek]

This looks great to me! I tested it with the os2.py example script, and it seems to be working great too :-)

[dsnopek]

Cherry-picked for 4.2 in PR #1410

[dsnopek]

Cherry-picked for 4.1 in PR #1411