CMake updates for VS2015/cmake 3.12 onwards

cmake 3.12 adds MSVC_TOOLSET_VERSION to define the version of MSVC toolchain in use
Use that if present instead of if-elseif chain

code/CMakeLists.txt

@@ -1037,6 +1037,10 @@ ENDIF (ASSIMP_BUILD_NONFREE_C4D_IMPORTER)
 
 if( MSVC )
   # in order to prevent DLL hell, each of the DLLs have to be suffixed with the major version and msvc prefix
+  # CMake 3.12 added a variable for this
+  if(MSVC_TOOLSET_VERSION)
+    set(MSVC_PREFIX "vc${MSVC_TOOLSET_VERSION}")
+  else()
   if( MSVC70 OR MSVC71 )
     set(MSVC_PREFIX "vc70")
   elseif( MSVC80 )
@@ -1051,9 +1055,12 @@ if( MSVC )
     set(MSVC_PREFIX "vc120")
   elseif( MSVC14 )
     set(MSVC_PREFIX "vc140")
+  elseif( MSVC15 )
+    set(MSVC_PREFIX "vc141")
   else()
     set(MSVC_PREFIX "vc150")
   endif()
+  endif()
   set(LIBRARY_SUFFIX "${ASSIMP_LIBRARY_SUFFIX}-${MSVC_PREFIX}-mt" CACHE STRING "the suffix for the assimp windows library")
 endif()