Support clang-cl on Windows

clang-cl is an MSVC-like driver for clang on Windows. It needs MSVC style options and CMake sets MSVC when it is being used.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b1d072c..eb19a29 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,7 +38,7 @@
 endif(NOT CMAKE_CXX_STANDARD)
 
 # https://github.com/izenecloud/cmake/blob/master/SetCompilerWarningAll.cmake
-if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+if(MSVC)
   # Use the highest warning level for Visual Studio.
   set(CMAKE_CXX_WARNING_LEVEL 4)
   if(CMAKE_CXX_FLAGS MATCHES "/W[0-4]")
@@ -55,7 +55,7 @@
   # Disable RTTI.
   string(REGEX REPLACE "/GR" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /GR-")
-else(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+else(MSVC)
   # Use -Wall for clang and gcc.
   if(NOT CMAKE_CXX_FLAGS MATCHES "-Wall")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall")
@@ -85,7 +85,7 @@
   # Disable RTTI.
   string(REGEX REPLACE "-frtti" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
-endif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+endif(MSVC)
 
 # BUILD_SHARED_LIBS is a standard CMake variable, but we declare it here to make
 # it prominent in the GUI.