CMake fixes

[foonathan]

Just two fixes to use the library better as CMake subproject (add_subdirectory()).

1. Export the targets for installation. This will create a file generating an IMPORT target, required for e.g. find_package() and allows linking the cmark targets to your own targets if your own targets are installed.
2. Remove a developer warning by suppressing the policy (see comments).

[jgm]

Is this really supposed to be lib/cmake?

[jgm]

Also note the comment at line 49 of src/CMakeLists.txt.

[foonathan]

CMake has a couple of places where it searches for config files generated by the install(EXPORT) command. /lib/cmake is one of them, useful if your project doesn't add its own subfolder for the libraries (like this here does).

I can change that to ${LIB_INSTALL_DIR}/cmake, didn't see the comment.

[foonathan]

When is this PR going to be merged?

[jgm]

I'm just worried about assuming that the build machine will always have a $LIB_INSTALL_DIR/cmake directory.

Is that universally true, even on Windows e.g.?

[foonathan]

The common CMake convention is that you install and export libraries under lib/project. But to suppport installing to lib directly you can also export under lib/cmake.

And the documentation for find_package() lists <prefix>/lib/cmake. It doesn't assume that the project has such a directory, it will be created when needed (afaik).

[jgm]

OK, I've merged now!

[foonathan]

Thanks! If you run into any CMake issues related to that, I'll be happy to help.