cmake: Rename PNG_EXECUTABLES to PNG_TOOLS

For the sake of consistency with a recent addition to the configure
script, the option name PNG_EXECUTABLES (introduced in libpng-1.6.38)
shall become PNG_TOOLS.

PNG_EXECUTABLES is still maintained as a deprecated option, allowing
the applications that use it to be built without modification, but a
deprecation warning will be issued.
1 file changed