[SCons] Platform agnostic default toolchain (GNU).

Create the SCons Environment with an empty PLATFORM variable to force
the default tools to use the GNU toolchain.

Platform specific toolchains are then setup in our custom tools.

tools/windows.py

@@ -23,7 +23,12 @@ def generate(env):
         elif env["arch"] == "x86_32":
             env["TARGET_ARCH"] = "x86"
         env["is_msvc"] = True
+
+        # MSVC, linker, and archiver.
         msvc.generate(env)
+        env.Tool("mslib")
+        env.Tool("mslink")
+
         env.Append(CPPDEFINES=["TYPED_METHOD_BIND", "NOMINMAX"])
         env.Append(CCFLAGS=["/EHsc"])
         env.Append(LINKFLAGS=["/WX"])