Disable install rule if robin-map is a subproject

[SachinVin]

Hello,
Recently a user reported that robin-map is installed when make install is used with our project.
A check to disable install whe robin-map is a subproject, would be great.

Examples of how a couple of other projects do it -
SDL: https://github.com/libsdl-org/SDL/blob/main/CMakeLists.txt#L3349

Catch2: https://github.com/catchorg/Catch2/blob/devel/CMakeLists.txt#L111

Thanks!

[Tessil]

Thank you for the report. I created a PR to fix the issue, could you take a look to check it works for you?

[SachinVin]

Thanks, that looks good!

[Tessil]

Thanks, I merged the change.