Create target for installation of libraries and headers into file system

[yourslab]

This adds the feature requested in issue #309, #1344, and #1424. It works by creating targets for each individual library and platform abstractions that can then be installed into any directory of a file system.

For instance, the following commands will install only the coreMQTT and coreHTTP libraries in the default system header and library path:

cmake .. -DINSTALL_LIBS="MQTT;HTTP" -DINSTALL_PLATFORM_ABSTRACTIONS=0 -DINSTALL_TO_SYSTEM=1
sudo make install

[ 26%] Built target aws_iot_mqtt
[ 46%] Built target aws_iot_http
Install the project...
-- Install configuration: ""
-- Installing: /usr/local/lib/libaws_iot_http.so
-- Installing: /usr/local/lib/libaws_iot_mqtt.so
-- Installing: /usr/local/include/aws/core_http_client.h
-- Installing: /usr/local/include/aws/core_http_config_defaults.h
-- Installing: /usr/local/include/aws/transport_interface.h
-- Installing: /usr/local/include/aws/http_parser.h
-- Installing: /usr/local/include/aws/core_mqtt.h
-- Installing: /usr/local/include/aws/core_mqtt_config_defaults.h
-- Installing: /usr/local/include/aws/core_mqtt_serializer.h
-- Installing: /usr/local/include/aws/core_mqtt_state.h
-- Installing: /usr/local/include/aws/transport_interface.h

Once installed, the files can be easily linked and included as a system header:

#include <aws/core_mqtt.h>
#include <aws/core_http_client.h>

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[aggarw13]

How is the header installation path accounted for?

[yourslab]

Yes, it is possible to also install the headers in this file. However, what ends up happening is that the make install output will no longer group libraries and headers together. To fix that, the headers are installed here instead:
https://github.com/aws/aws-iot-device-sdk-embedded-C/pull/1519/files/96807066b1b35fd543d45f6a57063325cc28dd69#diff-7021d478f66d585e3f9025b7212b1ca40b1db63995592041e28b5dd819c06e53R154-R157
Actually, I did want to also install all libraries in this file but it is not backwards compatible with older versions of CMake.

[aggarw13]

install all libraries in this file but it is not backwards compatible with older versions of CMake.

What aspect about installing all libraries in the file is not backwards compatible with older version of CMake?

[yourslab]

The cmake file that calls add_library() for a target is also where install() needs to be called for that target in older versions of cmake.

[aggarw13]

If PKCS11 requires a config file, then the user should be required to pass it instead of the one from our demos being used (imo).

I think it would be important to support selective library installation so that the users that do not want to install PKCS11 and also don't want to provide custom configs for any of the other libraries they want, they don't have to be forced to provide custom config for PKCS11

[yourslab]

I think it's fine if it is using the configs from the demo as those configs are specific to POSIX. I am supporting selective library installation though.

[aggarw13]

Should we support building and installing libraries as static archives if user wants? If that increases scope of this PR, it can be done in a follow-up PR.

[yourslab]

That is already a feature of CMake and can be specified with -DBUILD_SHARED_LIBS=0

[yourslab]

test

[aggarw13]

Where is the ota_pal library built?

[yourslab]

In platform/posix/ota_pal, and it is an implementation specific to POSIX.