debian: fix linking in emacsen-install script

When upgrading emacs, the install file can (apparently) be called with
the lisp files already linked into the flavour specific
directories. In this case we should not fail.

debian/notmuch-emacs.emacsen-install

@@ -31,7 +31,7 @@ echo install/${PACKAGE}: byte-compiling for ${FLAVOR}
 # Create symlinks to the .el files (see section 6E in debian-emacs
 # polcy).  This makes complation easy, and also allows find-function
 # and find-library to work properly.
-(cd ${elc_dir} && cp -s ${el_dir}/*.el .)
+(cd ${elc_dir} && ln -sf ${el_dir}/*.el .)
 
 # Byte compile them
 (cd ${elc_dir}