configure: Add --with-gperf option

On OS X, Apple supply an old gperf (3.0.3) with xcode and
xcode commandline tools which causes build failures:

./zconf.hash.c:183:17: error: expected expression
{offsetof(struct kconf_id_strings_t, kconf_id_strings_str2),

.. upgrading to gperf 3.0.4 was sufficient to fix this,
so this option allows the user to specify the gperf
program that they wish to use.

To install gperf 3.0.4 from homebrew, I did:

brew tap homebrew/dupes
brew install homebrew/dupes/gperf

.. then passed --with-gperf=$BREWFIX/Cellar/gperf/3.0.4/bin/gperf
to configure

Signed-off-by: Ray Donnelly <mingw.android@gmail.com>
Message-Id: <CAOYw7dtCmcJ9WiqmQ81MmZeRPcV-tDOqe9=kRDW4uQGuZNd2Ng@mail.gmail.com>
Patchwork-Id: 274892

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

     ./configure --help

 If you are using a development snapshot, you'll have to

 create the configure script, first. Just run:

     ./bootstrap

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://crosstool-ng.org/

 Aloha!