Revisit NUT CI build on CentOS 6

[jimklimov]

A question on mailing list was raised about testing NUT for someone still running CentOS 6, so here goes... spoils of experimental reconnaissance:

"Surprisingly, it works!" (mostly regarding feature/dependency availability)

With container setup as documented in this PR and downshift to allow autoconf-2.63 (also in the PR), ./ci_build.sh built and successfully NIT-tested the codebase (without C++11 support due to old GCC-4.4.x; clang-3.4.x refuses to use system-provided C++ headers).

Although the older python2.6 is the only one available on the system, PyNUT successfully participated in the test; the NUT-Monitor GUI was not tried (headless system).

Java 11 was not available for the platform, so for the time being it can not directly run a Jenkins agent and will not be part of regular NUT CI farm testing.

NUT Configuration summary:
==========================

* build serial drivers: yes
* build USB drivers:    yes (libusb-0.1)
* build neon based XML driver:  yes
* enable Avahi support: yes
* build Powerman PDU client driver:     yes
* build Modbus driver:  yes
* build IPMI driver:    yes (FreeIPMI)
* build Mac OS X meta-driver:   no
* build i2c based drivers:      no
* enable SSL support:   yes (OpenSSL)
* enable libwrap (tcp-wrappers) support:        yes
* enable libltdl (Libtool dlopen abstraction) support:  yes
* build nut-scanner:    yes
* build CGI programs:   yes
* install NUT-Monitor desktop application:      no
* install PyNUT binding module: no
* build and install documentation:      skip
* build specific documentation format(s):       no
* build and install the development files:      yes
* consider basic systemd support:       no
* build with tighter systemd support:   no
* build C++11 codebase (client library, etc.):  no
* build C++ tests with CPPUNIT: no
* build SNMP drivers:   yes
* build SNMP drivers with statically linked lib(net)snmp:       no
* User to run as:       nobody
* Group of user to run as:      nobody

NUT Paths:
----------

* Default installation prefix path:     /usr/local/ups
* State file path:      /var/state/ups
* Unprivileged PID file path:   /var/state/ups
* Privileged PID file path:     /run
* Driver program path:  /usr/local/ups/bin
* CGI program path:     /usr/local/ups/cgi-bin
* HTML file path:       /usr/local/ups/html
* Config file path:     /usr/local/ups/etc
* Data file path:       /usr/local/ups/share
* Tool program path:    /usr/local/ups/bin
* System program path:  /usr/local/ups/sbin
* System library path:  /usr/local/ups/lib
* System exec-library path:     /usr/local/ups/libexec

NUT Build/Target system info:
-----------------------------

* host env spec we run on:      x86_64-unknown-linux-gnu
* host env spec we built on:    x86_64-unknown-linux-gnu
* host env spec we built for:   x86_64-unknown-linux-gnu
* host OS short spec we run on: x86_64-linux-gnu
* host OS short spec we built on:       x86_64-linux-gnu
* host OS short spec we built for:      x86_64-linux-gnu

NUT Compiler settings:
----------------------

* CC            : gcc
* CFLAGS        : -isystem /usr/local/include -g -O2 -Wno-reserved-identifier -Wno-unknown-warning-option -std=gnu99 -Wall -Wsign-compare -Wno-error
* CXX           : g++
* CXXFLAGS      : -isystem /usr/local/include -g -O2 -Wno-reserved-identifier -Wno-unknown-warning-option -Wno-error
* CPP           : gcc -E
* CPPFLAGS      :

:; cat /etc/redhat-release
CentOS release 6.10 (Final)

:;  gcc --version
gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-23)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.