Fix #3124

[NK-Nikunj]

Fixes #3124

Proposed Changes

• Added CMakeLists.txt in examples/hello_world_component specific only to external builds. This CMakeLists.txt will not be executed while running cmake for building hpx.
• Added cmake script in circle.yml to integrate the hello_world_component into CI build.

Edit

• Fixed a typo in documentation for "using_hpx_pkgconfig".

[hkaiser]

@NK-Nikunj This PR contains the same changes as #3176. Should we close that other one without merging or will you remove that commit from this one?

[NK-Nikunj]

@hkaiser I forgot to see that commit to #3176 is also added here. Since this commit has been added to this one as well, I feel you can close #3176. I will edit the proposed changes above to include the changes done in #3176 as well.

[msimberg]

@NK-Nikunj The hello world component is failing during cmake configuration (you can see the output by clicking on details on the circleci entry). I believe you need to change HPX_DIR to ../build/lib/cmake/HPX in order for HPX to be found correctly (see docs).

[NK-Nikunj]

@msimberg Thank you!

[hkaiser]

@NK-Nikunj have you deleted all of the actual code now? Also, please note that if you rely on pkg-config, this has to be enabled for Linux systems only (or if pkg-config was found by cmake).

[NK-Nikunj]

@hkaiser, @sithhell told me to build hello_world_component to test pkg-config instead of the test code that was given in the src. That's why I deleted the src folder.

[hkaiser]

To re-iterate: the purpose of the original example was to demonstrate how an external program depending on HPX can be built. We ran into issues as at some point the example was not compiling anymore. #3124 proposed to solve this by adding the code to the CI system, ensuring that any future changes wouldn't break the example.

Now, this PR not only proposes to build another (unrelated) example, it also proposes to remove the original example source code (sorry if I misunderstand things). Could you please explain your rationale in more detail?

[NK-Nikunj]

@hkaiser I think you have misunderstood what I have proposed. When I asked @sithhell for implementation, he told me that the previous example ( the one that was in src directory ) was being built externally. He told me to build the hello_world_component from that build.

What I have done is simply redirect the tests/unit/build/CMakelists.txt to invoke examples/hello_world_component/CMakelists.txt instead of the tests/unit/build/src/CMakelists.txt. Also, I have changed the Makefile accordingly so that hello_world_client can be built. The hello_world_component cmake can also be invoked by a user if he wishes to build hello_world_component by also passing HPX_DIR into cmake command.

[hkaiser]

LGTM now, thanks!