Build conda based kits for SLC5

Adds a special platform called linux-64@glibc-2.5 which contains a Python 3.6 interpreter which has support for CentOS 5. It feels cleaner to base these kits on the glibc version rather than the OS.

I was then going to suggest using a script like this for detecting the host OS:

#!/bin/bash
###############################################################################
# (c) Copyright 2018-2019 CERN                                                #
#                                                                             #
# This software is distributed under the terms of the GNU General Public      #
# Licence version 3 (GPL Version 3), copied verbatim in the file "LICENSE".   #
#                                                                             #
# In applying this licence, CERN does not waive the privileges and immunities #
# granted to it by virtue of its status as an Intergovernmental Organization  #
# or submit itself to any jurisdiction.                                       #
###############################################################################
set -euo pipefail
IFS=$'\n\t'


# Known platforms from: https://conda-static.anaconda.org/conda-forge/index.html
# linux-32
# linux-64
# linux-aarch64
# linux-ppc64le
# osx-64


if [ -z "${force_host_os:-}" ] ; then
  _os=$(uname -s)
  case $_os in
    'Linux')
      _os='linux'
      ;;
    'Darwin')
      _os='osx'
      ;;
    *) ;;
  esac

  _arch=$(uname -m)
  case $_arch in
    'x86_64')
      _arch='64'
      ;;
    'ppc64')
      _arch='ppc64le'
      ;;
    'aarch64')
      _arch='aarch64'
      ;;
    'i686')
      _arch='32'
      ;;
    *) ;;
  esac

  host_os="${_os}-${_arch}"

  # Fix up for systems with really old glibc versions
  if [[ "${host_os}" == "linux-64" ]]; then
    _libc_type=$(getconf GNU_LIBC_VERSION | cut -d " " -f 1)
    _libc_major=$(getconf GNU_LIBC_VERSION | cut -d " " -f 2 | cut -d '.' -f 1)
    _libc_minor=$(getconf GNU_LIBC_VERSION | cut -d " " -f 2 | cut -d '.' -f 2)
    if [ "${_libc_type}" == "glibc" ] && [ "${_libc_major}" -eq "2" ]; then
      if [ "${_libc_minor}" -lt "12" ]; then
        # CentOS 5
        host_os="linux-64@glibc-2.5"
      fi
    fi
  fi
else
  (>&2 echo "warning: overriding host os detection (using ${force_host_os})")
  _os=${force_host_os}
fi

echo "${host_os}"