Projucer: Check for libc++ version instead of macOS SDK version when working around a live-build error

modules/juce_core/native/juce_mac_ClangBugWorkaround.h

@@ -33,9 +33,9 @@
  #define _Nullable
  #define _Nonnull
 
- // In the 10.14 SDK these methods are defined in the <functional> header, which we don't compile
- // in the live-build engine, so we'll define them here
- #if defined (MAC_OS_X_VERSION_10_14) && MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_14
+ // In later versions of libc++ these methods are defined in the functional header,
+ // which we don't compile in the live-build engine, so we'll define them here
+ #if defined (_LIBCPP_VERSION) && _LIBCPP_VERSION >= 7000
   #include <memory>
 
   namespace std { inline namespace __1 {