Switch to a minumim version constraint for the MSVC version checks

Summary:
Just require that MSVC is at least MSVC 2017

Closes: https://github.com/facebook/folly/issues/949

Reviewed By: yfeldblum

Differential Revision: D10355264

fbshipit-source-id: 52ac7489200285f06c5f4fb3d176f6427974b163

CMake/FollyCompilerMSVC.cmake

@@ -89,10 +89,9 @@ function(apply_folly_compile_options_to_target THETARGET)
       /Zc:threadSafeInit # Enable thread-safe function-local statics initialization.
       /Zc:throwingNew # Assume operator new throws on failure.
 
+      /permissive- # Be mean, don't allow bad non-standard stuff (C++/CLI, __declspec, etc. are all left intact).
       /std:${MSVC_LANGUAGE_VERSION} # Build in the requested version of C++
 
-      # This is only supported by MSVC 2017
-      $<$<BOOL:${MSVC_IS_2017}>:/permissive-> # Be mean, don't allow bad non-standard stuff (C++/CLI, __declspec, etc. are all left intact).
     PRIVATE
       /bigobj # Support objects with > 65k sections. Needed due to templates.
       /favor:${MSVC_FAVORED_ARCHITECTURE} # Architecture to prefer when generating code.
@@ -188,13 +187,6 @@ function(apply_folly_compile_options_to_target THETARGET)
       /wd4701 # Potentially uninitialized local variable used.
       /wd4702 # Unreachable code.
 
-      # MSVC 2015 only:
-      $<$<BOOL:${MSVC_IS_2015}>:
-        /wd4268 # Static/global data initialized with compiler generated default constructor fills the object with zeros.
-        /wd4510 # Default constructor was implicitly defined as deleted.
-        /wd4814 # In C++14 'constexpr' will not imply 'const'.
-      >
-      
       # These warnings are disabled because we've
       # enabled all warnings. If all warnings are
       # not enabled, we still need to disable them

CMakeLists.txt

@@ -49,17 +49,11 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
     message(FATAL_ERROR "Folly requires a 64bit target architecture.")
   endif()
 
-  if (MSVC_VERSION GREATER_EQUAL 1910 AND MSVC_VERSION LESS 1920)
-    set(MSVC_IS_2015 OFF)
-    set(MSVC_IS_2017 ON)
-  elseif (MSVC_VERSION EQUAL 1900)
-    set(MSVC_IS_2015 ON)
-    set(MSVC_IS_2017 OFF)
-  else()
+  if (MSVC_VERSION LESS 1900)
     message(
       FATAL_ERROR
       "This build script only supports building Folly on 64-bit Windows with "
-      "Visual Studio 2015 or Visual Studio 2017. "
+      "at least Visual Studio 2017. "
       "MSVC version '${MSVC_VERSION}' is not supported."
     )
   endif()