SCons: Fix handling of platform-specific tools, notably mingw

Add optional `detect.py` `get_tools` method to let platforms register SCons
tools they need.

This helps move this logic out of SConstruct, keeping platforms more self
contained, and helping thirdparty platforms define their own custom tools.

This logic was also unreliable (the `use_mingw` one would only work if
passed manually on the command line, not in e.g. `get_flags`).

Co-authored-by: Thaddeus Crews <repiteo@outlook.com>

platform/web/detect.py

@@ -28,6 +28,11 @@ def can_build():
     return WhereIs("emcc") is not None
 
 
+def get_tools(env: "SConsEnvironment"):
+    # Use generic POSIX build toolchain for Emscripten.
+    return ["cc", "c++", "ar", "link", "textfile", "zip"]
+
+
 def get_opts():
     from SCons.Variables import BoolVariable