Is it necessary to include the GPL / LGPL license text to every distributed copy of my program? Or can I just put a link in the
Yes, because the link might become invalid in 5 or 10 years.
Yes, this is necessary. You cannot just include a link pointing to the license text. The reason is clearly stated in the GNU FAQ:
Why does the GPL require including a copy of the GPL with every copy of the program?
Including a copy of the license with the work is vital so that everyone who gets a copy of the program can know what his rights are.
It might be tempting to include a URL that refers to the license, instead of the license itself. But you cannot be sure that the URL will still be valid, five years or ten years from now. Twenty years from now, URLs as we know them today may no longer exist.
The only way to make sure that people who have copies of the program will continue to be able to see the license, despite all the changes that will happen in the network, is to include a copy of the license in the program.