platform expressions: windows should also be true for mingw systems (#113)

src/vcpkg/platform-expression.cpp

@@ -389,7 +389,8 @@ namespace vcpkg::PlatformExpression
                         case Identifier::arm64: return true_if_exists_and_equal("VCPKG_TARGET_ARCHITECTURE", "arm64");
                         case Identifier::windows:
                             return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "") ||
-                                   true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "WindowsStore");
+                                   true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "WindowsStore") ||
+                                   true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "MinGW");
                         case Identifier::mingw: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "MinGW");
                         case Identifier::linux: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Linux");
                         case Identifier::osx: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Darwin");