Check if TARGET_OS_OSX is defined before evaluating its value

[nilFinx]

Solves a case where APPLE is defined, but TARGET_OS_OSX isn't, causing a compile error. Happens on my OS X 10.9 setup with MacPorts Clang 16 instead of the Xcode Clang 6.

Compile log before fix:

error: 'TARGET_OS_OSX' is not defined, evaluates to 0 [-Werror,-Wundef-prefix=TARGET_OS_]
#elif defined(__APPLE__) && TARGET_OS_OSX && defined(IMGUI_ENABLE_OSX_DEFAULT_CLIPBOARD_FUNCTIONS)
                            ^
1 error generated.

[ocornut]

How could it be undefined at the top of imgui.cpp has:

// [Apple] OS specific includes
#if defined(__APPLE__)
#include <TargetConditionals.h>
#endif

Can you clarify?

I am not confident having random tests without validating the root cause.
By the logic of your PR, there are 27 other uses of TARGET_OS_OSX in other code and they should all be modified.

Could you investigate why MacPorts Clang 16 doesn't have those inside TargetConditionals.h ?