Clean up CMakeLists.txt
* Rename the target `png` to `png_shared`. The main targets are now
named `png_shared`, `png_static` and `png_framework`.
* Rename the function symbol_prefix() to find_symbol_prefix().
* Rename the prefix `S` to `_SYM` in the function create_symlink().
* Rewrite the precondition checks in the function create_symlink().
* Add precondition checks to the function generate_copy().
* Delete the vestigial property CLEAN_DIRECT_OUTPUT.
* Delete the variables PNG_LIB_NAME_STATIC and PNG_LIB_NAME_FRAMEWORK.
* Initialize and use PNG_LIB_TARGETS consistently as a list.
* Move all include() commands to the top of the file.
* Acknowledge a former contributor.
* Reformat.
1 file changed
