Fix standard library selection on OS X

For test program compile during configure stage when targeting 10.8 from 
>=10.15

CMakeLists.txt

@@ -25,6 +25,8 @@ if (APPLE)
   set(CMAKE_XCODE_GENERATE_SCHEME ON)
 	set(CMAKE_OSX_ARCHITECTURES "x86_64" CACHE STRING "")
   set(CMAKE_OSX_DEPLOYMENT_TARGET "10.8" CACHE STRING "")
+  #A consequence of targetting 10.8. Needs to be set globally from 10.15 onwards in order for the test program to compile successfully during configure 
+  string(APPEND CMAKE_CXX_FLAGS " -stdlib=libc++")
 endif()
 
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