complibs/ppl: update GMP location configuration argument for PPL v0.11 and later

'configure' for PPL 0.11 (and later) needs "--with-gmp-prefix" to
provide the location of the GMP toolkit; the previous switches were
"--with-libgmp-prefix" and "--with-libgmpxx-prefix".

The upstream log message is:

commit 08dfb6fea094f8c5a533575a3ea2095edce99a6d
Author: Roberto Bagnara <bagnara@cs.unipr.it>
Date: Sun Jul 12 21:39:46 2009 +0200

New configure option --with-gmp-prefix supersedes the (now removed)
options --with-libgmp-prefix and --with-libgmpxx-prefix.

Link: http://www.cs.unipr.it/git/gitweb.cgi?p=ppl/ppl.git;a=commit;h=08dfb6fea094f8c5a533575a3ea2095edce99a6d

Since PPL's 'configure' ignores unknown switches, we use all three so we
don't have to conditionalize the ppl.sh build script itself.

Signed-Off-By: Anthony Foiani <anthony.foiani@gmail.com>
(transplanted from 4f0c4fb572e2862c24b28e8d27ce7e9cb9adba65)

 # Options specific to crosstool-NG acting as a backend

 config IS_A_BACKEND

     string

     option env="CT_IS_A_BACKEND"

 config BACKEND

     bool

     default y if IS_A_BACKEND =  "y" || IS_A_BACKEND =  "Y"

     default n if IS_A_BACKEND != "y" && IS_A_BACKEND != "Y"

 config BACKEND_ARCH

     string

     option env="CT_BACKEND_ARCH"

 config BACKEND_KERNEL

     string

     option env="CT_BACKEND_KERNEL"

 config BACKEND_LIBC

     string

     option env="CT_BACKEND_LIBC"