Backport LLVM's r341717 "Fix flags used to compile benchmark library …

…with clang-cl" (#673)

`MSVC` is true for clang-cl, but `"${CMAKE_CXX_COMPILER_ID}" STREQUAL
"MSVC"` is false, so we would enable -Wall, which means -Weverything
with clang-cl, and we get tons of undesired warnings.

Use the simpler condition to fix things.

Patch by: Reid Kleckner @rnk

CMakeLists.txt

@@ -94,7 +94,7 @@ if (BENCHMARK_BUILD_32_BITS)
   add_required_cxx_compiler_flag(-m32)
 endif()
 
-if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+if (MSVC)
   # Turn compiler warnings up to 11
   string(REGEX REPLACE "[-/]W[1-4]" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W4")