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 NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.