configure: do not use '@' in version string

#0a0410dd0cb0 broke #0cc4d6352c3e. This patch fixes this, which fixes the build
of gcc/glibc/gdb manuals.

Signed-off-by: "Benoît Thébaudeau" <benoit.thebaudeau@advansee.com>

 #!/bin/sh

 call_to=@@DEFAULT_LD@@

 case "${CTNG_LD_IS}" in

     bfd)    call_to=bfd;;

     gold)   call_to=gold;;

 esac

 exec "${0}.${call_to}" "$@"

 exit $?