Enable parent project to specify dependency directory

[lolsborn]

This change allows parent projects to determine where other dependencies should live by adding the following:

set(DEPENDENCY_DIR "vendors")

The reason for this is that the aws-iot-device-sdk-cpp library is likely already being installed along with other project dependencies. Currently aws-iot creates another directory at the top level of the parent project for it's dependencies. This is probably not desired for projects with existing dependencies and existing directory structure for third-party libraries.

Another option may be to simply ensure that the aws-iot dependencies are downloading in a subdirectory relative to itself and not the parent project.

This patch was made on top of @larsonmpdx's branch, but is not dependent upon it. I am happy to separate it out if necessary.

[chaurah]

Hi @lolsborn,
Thank you for this contribution. You don't need to separate it out. We have already merged @larsonmpdx's patch internally. Your patch will also be included along with the next release of the SDK. Please do let us know if you have further suggestions.

Rahul

[chaurah]

This pull request has been merged in v1.1 release of the SDK.
Rahul