CMake: include FollyCompilerMSVC if the compiler is MSVC (#1421)

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1421

Reviewed By: yfeldblum

Differential Revision: D23023218

Pulled By: Orvid

fbshipit-source-id: 7cc182319dece37b9f2db42958717fa70b526025

CMakeLists.txt

@@ -103,7 +103,7 @@ set(FOLLY_HAVE_PTHREAD "${CMAKE_USE_PTHREADS_INIT}")
 list(APPEND CMAKE_REQUIRED_LIBRARIES Threads::Threads)
 list(APPEND FOLLY_LINK_LIBRARIES Threads::Threads)
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
+if(MSVC)
   include(FollyCompilerMSVC)
 else()
   include(FollyCompilerUnix)