Ignore 32 bit build option when using MSVC (#638)

CMakeLists.txt

@@ -16,7 +16,11 @@ option(BENCHMARK_ENABLE_TESTING "Enable testing of the benchmark library." ON)
 option(BENCHMARK_ENABLE_EXCEPTIONS "Enable the use of exceptions in the benchmark library." ON)
 option(BENCHMARK_ENABLE_LTO "Enable link time optimisation of the benchmark library." OFF)
 option(BENCHMARK_USE_LIBCXX "Build and test using libc++ as the standard library." OFF)
-option(BENCHMARK_BUILD_32_BITS "Build a 32 bit version of the library." OFF)
+if(NOT MSVC)
+  option(BENCHMARK_BUILD_32_BITS "Build a 32 bit version of the library." OFF)
+else()
+  set(BENCHMARK_BUILD_32_BITS OFF CACHE BOOL "Build a 32 bit version of the library - unsupported when using MSVC)" FORCE)
+endif()
 option(BENCHMARK_ENABLE_INSTALL "Enable installation of benchmark. (Projects embedding benchmark may want to turn this OFF.)" ON)
 
 # Allow unmet dependencies to be met using CMake's ExternalProject mechanics, which