MSYS: Add support for running under MSYS runtime environment

Modules/CMakeDetermineSystem.cmake

Modules/CMakePlatformId.h.in

Modules/CMakeSystemSpecificInformation.cmake

Modules/FindThreads.cmake

Modules/Platform/CYGWIN.cmake

Modules/Platform/MSYS-Clang-C.cmake

Modules/Platform/MSYS-Clang-CXX.cmake

Modules/Platform/MSYS-Determine-CXX.cmake

Modules/Platform/MSYS-GNU-C.cmake

Modules/Platform/MSYS-GNU-CXX.cmake

Modules/Platform/MSYS-GNU-Fortran.cmake

Modules/Platform/MSYS-GNU.cmake

Modules/Platform/MSYS-windres.cmake

Modules/Platform/MSYS.cmake

Source/CMakeLists.txt

Source/Checks/cm_cxx_features.cmake

Source/Modules/FindLibUUID.cmake

Source/cmExtraEclipseCDT4Generator.cxx

Source/cmSystemTools.cxx

Tests/CMakeLists.txt