[cmake] Allow config.guess to be run with MSYS on Windows

Summary:
With r363420, config.guess can no longer be run with MSYS on Windows and this
patch should be able to fix this particular case.

Reviewers: compnerd

Reviewed By: compnerd

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63834

llvm-svn: 364485

llvm/cmake/modules/GetHostTriple.cmake

@@ -15,7 +15,7 @@ function( get_host_triple var )
       set( value "i686-pc-windows-gnu" )
     endif()
   else( MSVC )
-    if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+    if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows AND NOT MSYS)
       message(WARNING "unable to determine host target triple")
     else()
       set(config_guess ${LLVM_MAIN_SRC_DIR}/cmake/config.guess)