[png-mng-implement] CMake symlink creation and minimum needed version

[Clifford Y. <cli...@gm...>]

Now that even Debian old-old-stable has moved to CMake 2.8.9, would it
be possible to think about bumping the minimum version of CMake needed
for libpng?  If so, I've prepared a patch (attached) which addresses
the creation of symlinks in light of the deprecation of the LOCATION
property.  (Newer CMake versions are getting increasingly vocal about
pointing out the need to shift away from using LOCATION, but to do so
for this feature we need the generator expressions introduced in
2.8.8.)

Feedback/comments welcome - if we truly need to support pre 2.8.8
CMake I can try to make a hybrid version that handles both methods,
but I'm reluctant to complicate the logic - hopefully any system so
old it doesn't have 2.8.8 can just use the autotools build anyway...

Thanks,
CY