Changes to bootstrap and build with the MinGW environment.

* Define the OS as NT as this seems more reasonable than having MINGW as the OS (after all MinGW is the toolset not the OS).
* Add detection of one common install location for MinGW.


[SVN r18073]

tools/build

@@ -1 +1 @@
-Subproject commit 19d9f9a467cfd3535ec1779d4c9e5cf666952274
+Subproject commit 09dee4da89ae62244961d5c8753ffeb07c4cd052