[GPL] No linking with proprietary programs: where?

[Stephane Bortzmeyer <bortzmeyer@pasteur.fr>]

[Please Cc: me the replies, i'm not on the list.]

We all know that the GPL prevents linking with proprietary stuff (see the 
KDE/Qt thread). And we all take this for granted.

Looking for the precised wording, I was very surprised to be unable to find 
it. Where in the GPL is such restriction written? I just find this line at the 
end, in "How to Apply These Terms to Your New Programs" where I wouldn't have 
search:

This General Public License does not permit incorporating your program into 
proprietary programs. [Where is it written? S. B.]  If your program is a 
subroutine library, you may consider it more useful to permit linking 
proprietary applications with the library. If this is what you want to do, use 
the GNU Library General Public License instead of this License.