Check for undefined TARGET_OS_*

[jmroot]

Prevents some versions of clang from erroring out like this:

error: 'TARGET_OS_TV' is not defined, evaluates to 0 [-Werror,-Wundef-prefix=TARGET_OS_]

[icculus]

We might need this fix in the real SDL headers too.

[slouken]

Yup, I believe we do.

[sezero]

Those aren't actually in real SDL-1.2 headers.

[sezero]

Pushed https://github.com/libsdl-org/sdl12-compat/commit/c1eb2fecf9c5fc05e2efce0e2a135cd7474a4874 as a follow-up.