Detect bits when building with MinGW

(cherry picked from commit 460f030b738eb055d915e246a61f7740fe14f57b)

platform/windows/detect.py

@@ -306,7 +306,7 @@ def configure(env):
         mingw_prefix = ""
 
         if (env["bits"] == "default"):
-            env["bits"] = "32"
+            env["bits"] = "64" if "PROGRAMFILES(X86)" in os.environ else "32"
 
         if (env["bits"] == "32"):
             env.Append(LINKFLAGS=['-static'])