Add install target

[jyscao]

• this allows $ make install to also execute successfully
• without changing the output location of nuklear.so

Rationale

I've been trying to package up the love-nuklear project for Guix. However due to the generated Makefile not having an installation target, Guix produces an error when executing $ guix install love-nuklear, even though the binary was successfully built. I was able to get it to go through the full build-process without errors by modifying the install phase of Guix's cmake-build-system in the package definition. Though I figured it might be even better to make the change upstream so that the project's build process would be more robust in general.

Please let me know what you think, and ofc suggestions and improvements are welcomed.

[keharriso]

Code solves a problem and doesn't break anything. Thanks for another quality pull request.