Changes

Add CMake code to package and download pre-built libraries (#1106).

Exclude `./external` directories from source code management (#1106).

Use CMake's `ExternalProject` to build zlib and then package library
using new mechanism (#1106).

Tidy code.

Pre-build zlib for Linux and Windows (#1106).

Some minor cleaning up.

Various cleaning up (#1106).
- Simplified our versioning mechanism for zlib;
- Replaced 'external' with 'ext’;
- Moved the 'zlib' contents to 'ext/proj’;
- Changed the order of some parameters in CREATE_PACKAGE_FILE(), 
RETRIEVE_PACKAGE_FILE() and RETRIEVE_PACKAGE_FILE_FROM();
- Replaced ‘CHECKED_FILES’ with ‘SHA1_FILES’;
- Use our forked version of zlib;
- Updated the SHA-1 values for zlib, now that the zlib package is hosted
on opencor.ws;
- Various minor cleaning ups.

Updated our SHA-1 values (#1106).

Some minor cleaning up.

Make sure that a package is retrieved in the right directory (#1106).

Some minor cleaning up.

Some minor cleaning up.

Replaced 'DEPENDS' with 'DEPENDS_ON' in ADD_PLUGIN().

Some minor cleaning up.

Addressed an issue with Ninja with regards to copying OpenCOR.com.

Some minor cleaning up.

ADD_PLUGIN(): make sure that various commands are either executed
directly or through a custom target based on the plugin's situation
(#1106).

Some minor cleaning up.

Some minor cleaning up.

Some minor cleaning up.

Some minor cleaning up.

Cleaned up the CREATE_PACKAGE_FILE() macro (#1106).

Some minor cleaning up.

Cleaned up the CREATE_PACKAGE_FILE() macro (#1106).

Renamed CHECK_FILES() with CHECK_SHA1_FILES() (#1106).

Use CHECK_SHA1_FILES_OK rather than CHECK_SHA1_FILES_FAILED (#1106)
Kind of positive attitude… :)

Cleaned up CHECK_SHA1_FILES() (#1106).

Reverted our use of CHECK_SHA1_FILES() back in favour of CHECK_FILES().
Also introduced a convenience macro: CHECK_FILE() (#1106).

Some minor cleaning up.

Some minor cleaning up.

Some minor cleaning up.

Some minor cleaning up.

Cleaned up the RETRIEVE_PACKAGE_FILE_FROM() and
RETRIEVE_BINARY_FILE_FROM() macros (#1106).

Make sure that we have at least one file to check when calling the
RETRIEVE_PACKAGE_FILE_FROM() macro (#1106).

Make sure that we report the case where a file/package couldn't be
uncompressed (#1106).

Output to the console the RETRIEVE_PACKAGE_FILE() call that needs to be
made (#1106).

Make sure that we properly generate the information about the call to
the RETRIEVE_PACKAGE_FILE() macro (#1106).

Generate a package file that can be uploaded on GitHub (#1106).

Have a package file generated in our main build directory, so that it's
easier to find (#1106).

Getting ready to generate new zlib package binaries (#1106).

Retrieve our package files directly from GitHub (#1106).

Slight updates to our downloads pages (#1106).