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

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"