complibs/ppl: fix build when local system has libgmp already installed

Basically, the ppl configure script wasn't properly setting
CFLAGS/CXXFLAGS/LDFLAGS to point at the ctng-built gmp, so ppl tried to
use the system gmp headers instead.

This patch fixes that for all PPL versions supported by crosstools-ng.

Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
[yann.morin.1998@free.fr: slightly touch the commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <20130715230656.GB23382@shaftnet.org>
Patchwork-Id: 259300

 #!/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 $?