KDE4_ADD_KDEINIT_EXECUTABLE: Check the right variable for Mac.

Fix a regression introduced by 52ee7642, which copy-pasted some code
from the Windows block. It caused the executable created with this macro
to unconditionally build and depend on the last source file passed to
it, even though it's only supposed to be part of the kdeinit_$TARGET
target.

BUG:		334702
FIXED-IN:	4.13.2