STYLE: Simplify zlib external project

Added SLICER_ZLIB_LIBRARY_DIR CMake variable

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22219 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add SSL support

Having the application interacting with services or downloading resources
served over https protocol is now required. More particularly,
it became necessary to access resources stored on Github (extension
manager - issue #2354) or to download python packages from pip server
at run time.

The CMake option "Slicer_USE_PYTHONQT_WITH_OPENSSL" allowing to enable or
disable OpenSSL support has been introduced is set to OFF by default.

Building OpenSSL
================

On Windows, since Perl is required, OpenSSL binaries have been
generated and uploaded on kitware packages server. See [1]

The project allowing to re-generate the Windows OpenSSL archive is currently
hosted here [2].

On unix-like system, OpenSSL is built has part of the Superbuild. To ensure
is can be built in parallel, a patch is also applied [3].

Finally, since python is built with OpenSSL support, the python launcher
is now configured on all platform so that python can properly load the built
OpenSSL shared library when it is used to build project like numpy.

[1] http://packages.kitware.com/packages/application/view?applicationId=20
[2] https://gist.github.com/jcfr/6030240
[3] https://raw.github.com/Alexpux/Qt-builds/master/patches/openssl/openssl-1.0.1-parallel-build.patch

OpenSSL support in Qt, python, curl
===================================

Practically, it means that library like Qt, python, curl need to be
built with OpenSSL support.

This commit only address OpenSSL support for Python and Qt.

Curl support is not yet enabled.

For Qt OpenSSL support, Qt has to be built with the flags:
  -openssl
  -I /path/to/OpenSSL/include
  -L /path/to/OpenSSL/lib

On unix-like system, available binaries are already OpenSSL ready, Qt
has to be explicitly built only on windows. On windows, Qt will have to
be built.

Certificates
============

In case X.509v3 certificates are available on the system where Slicer is
executed, a certificate bundle "Slicer.crt" is also provided. The set of
scripts allowing to generate the bundle are provided by BLFS
(Beyond Linux From Scratch).
More details in Base/QTCore/Resources/Certs/README

Application integration
=======================

The Slicer core application will load the Slicer.crt bundle if needed.

qSlicerExtensionsInstallWidget now derives from a re-usable widget
named qSlicerWebWidget.

qSlicerSslTest has been added to check that "https" connection can
be established using Qt library.

Fixes #2998
Fixes #2355
Fixes #2354

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22220 3bd1e089-480b-0410-8dfb-8563597acbee