Allow FetchContent usage in external projects

[nicovanduijn]

Description

My project has an existing FreeRTOS Kernel and I attempted to include this project via CMake's FetchContent function as suggested by the Readme. In this process I stumbled over two minor details:

• Copy-paste error in the readme, suggesting to set defines for the FAT build instead of the TCP library
• No functionality to prevent the TCP library to pull in its own copy of the kernel, as other libraries have

Test Steps

• Set up a project with bare-bones FreeRTOS
• Consume this library via CMake's FetchContent as suggested by the readme

Checklist:

• [~] I have tested my changes. No regression in existing tests.
  -> I have obviously tested that this solution fixes my problem, but I'm unsure how I can test it against regressions in the upstream CI other than just opening this PR.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.
  -> No real code changes outside the build system, so this does not apply

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kstribrnAmzn]

Thank you for submitting this PR! I took a look over it and saw nothing wrong. I'll send this to the TCP experts on the team for their review.

[kstribrnAmzn]

CMake documentation (for the latest release) mentions

True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number (including floating point numbers). False if the constant is 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty string, or ends in the suffix -NOTFOUND.

We should keep default behavior to clone the FreeRTOS kernel. This default behavior is used in this now failing check. Maybe negate your variable to something like FREERTOS_PLUS_TCP_USE_LOCAL_FREERTOS. Then your condition could be...

if(NOT FREERTOS_PLUS_TCP_USE_LOCAL_FREERTOS)
    FetchContent_MakeAvailable(freertos_kernel cmock)
endif()

[nicovanduijn]

Great input, indeed this breaks existing builds. Thanks for the pointer.

I flipped the logic, hopefully CI passes now.

[archigup]

Perhaps it makes sense to check if we are the top level CMake project and only clone FreeRTOS kernel then?

[nicovanduijn]

I didn't know this was possible until I googled it just now. Is doing this considered best practice?

[nicovanduijn]

@archigup I've changed the logic as you suggested