Add --with-gperf=<PATH> 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
Remove another cp -aT

.. this time in 310-openssl.sh, as Darwin
doesn't support it either (FWIW, we won't
build openssl in crosstool-ng when Darwin
host and Darwin target, but someone might
want to for some other configuration.)