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/windows/detect.py

@@ -155,6 +155,13 @@ def detect_build_env_arch():
     return ""
 
 
+def get_tools(env: "SConsEnvironment"):
+    if os.name != "nt" or env["use_mingw"]:
+        return ["mingw"]
+    else:
+        return ["default"]
+
+
 def get_opts():
     from SCons.Variables import BoolVariable, EnumVariable
 
@@ -326,10 +333,6 @@ def setup_mingw(env: "SConsEnvironment"):
         print_error("No valid compilers found, use MINGW_PREFIX environment variable to set MinGW path.")
         sys.exit(255)
 
-    env.Tool("mingw")
-    env.AppendUnique(CCFLAGS=env.get("ccflags", "").split())
-    env.AppendUnique(RCFLAGS=env.get("rcflags", "").split())
-
     print("Using MinGW, arch %s" % (env["arch"]))