Fix __cplusplus macro for Visual Studio (#237)

* Fix __cplusplus macro for Visual Studio

* Upgrade GTest

* Force rebuild of GTest to build with C++17 onwards support

* Reinstate cmake layout

* Test if fetched gtest correctly builds for C++17

* Manually build Gtest to work around fetching the wrong build from the Conan Index Centre

* Attemp to use pre-built GTest if available

* Test fix is backward compatible with prior versions

* Revert to latest version

* Force building GTest on Windows is still required

cmake/compiler.cmake

@@ -30,6 +30,7 @@ target_link_libraries(MorpheusConfig
 
 target_compile_options(MorpheusConfig
     INTERFACE
+        $<$<CXX_COMPILER_ID:MSVC>:/Zc:__cplusplus> # https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
         $<$<CXX_COMPILER_ID:MSVC>:${MSVC_WARNINGS}>
         $<$<CXX_COMPILER_ID:GNU>:${GCC_WARNINGS}>
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:${CLANG_WARNINGS}>