CMake: Optional Install if Embedded

[ax3l]

When adding this library as embedded library with private "target link", e.g. only used inside private source files, the library does not need to be installed when the main project gets installed.

This adds an additional option JSON_Install similar to the test-build control switch in order to skip installing headers and CMake config files if requested.

Avoids using

add_subdirectory(path/to/nlohmann_json EXCLUDE_FROM_ALL)

which has further side-effects:
https://cmake.org/cmake/help/v3.0/command/add_subdirectory.html

cc @chuckatkins do you have an opinion on this?

[coveralls]

Coverage remained the same at 100.0% when pulling f049836 on ax3l:topic-installEmbed into 2f73a4d on nlohmann:develop.

[nlohmann]

Yes, I would also know what @chuckatkins thinks about this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ax3l]

Any thoughts, @chuckatkins ?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ax3l]

Hi @chuckatkins, sorry to bother you about this but I wonder if this is good practice or if you also just go with add_subdirectory(path/to/nlohmann_json EXCLUDE_FROM_ALL). The latter has more side-effects besides install-skipping.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ax3l]

ping @chuckatkins any idea?

[t-b]

@ax3l @nlohmann I just came up with the exact same patches. Should have checked the PRs first ;)

Can I help with something to get this merged?

[ax3l]

@t-b I am still wondering what other projects do, since this seems to be a common workflow that a simple EXCLUDE_FROM_ALL doesn't cut.
Besides that, we could merge it. It's in the worst-case just a project specific option we can control.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!

[t-b]

@nlohmann Thanks for merging.

@ax3l According to https://stackoverflow.com/a/51746390 most projects add an option to skip the installation.

[ax3l]

Good to know, so my intuition wasn't all off.

Thanks for merging!