configure: portability fixes - take 2

It is useful to sometime run on a strictly POSIX-compliant
shell... :-/

And it is sometime interesting to refresh Q-patches before
finishing the series... :-(

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 #!/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 $?