Don't specify standard library on macOS

libc++ is already the default and GCC can't even be used so there's
no circumstance where we'd need to explicitly tell Clang to use
libc++. I confirmed that even with this removed, libc++ headers are
still being used and found.

cmake/Macros.cmake

@@ -20,13 +20,8 @@ function(sfml_set_stdlib target)
         endif()
     endif()
 
-    if (SFML_OS_MACOSX)
     if(${CMAKE_GENERATOR} MATCHES "Xcode")
         sfml_set_xcode_property(${target} CLANG_CXX_LIBRARY "libc++")
-        else()
-            target_compile_options(${target} PRIVATE "-stdlib=libc++")
-            target_link_libraries(${target} PRIVATE "-stdlib=libc++")
-        endif()
     endif()
 endfunction()