SCons: Add option to toggle warnings (on by default)

All the warnings are factored out of the platform-specific files and moved to
SConstruct. Will have to check that it does not introduce regressions on some
platforms/compilers.

(cherry picked from commit 31107daa1a)

modules/freetype/SCsub

@@ -70,7 +70,6 @@ if (env['builtin_freetype'] != 'no'):
     # LIBS contains first SCons Library objects ("SCons.Node.FS.File object")
     # and then plain strings for system library. We insert between the two.
     inserted = False
-    print(env["LIBS"])
     for idx, linklib in enumerate(env["LIBS"]):
         if isinstance(linklib, basestring): # first system lib such as "X11", otherwise SCons lib object
             env["LIBS"].insert(idx, lib)