Missing compile option -lrt when target is Linux

[spacelan]

If not set, j2v8 may throw undefined reference to 'clock_gettime' at runtime

I added this option to https://github.com/eclipsesource/J2V8/blob/master/cmake/NodeJsUtils.cmake#L142 and solved this issue

set (njs_${config_name}_libs ${njs_libs} -lrt PARENT_SCOPE)

[irbull]

Some details about this flag here https://stackoverflow.com/questions/15821663/what-is-the-lrt-flag-in-gnu-make.

@drywolf Thoughts on this? do you know what platforms will need this flag?

[drywolf]

Looks like the node.js source has some references to the clock_gettime function, so it would make sense that it needs to be included.
@irbull I had another look at the Dockerfiles you added to test the linux/android cross-builds and there you also made reference to librt. Also for the Android build there seem to be a couple of additional libraries that are linked, but will not report any hard linker errors if they are not included.

I had a quick look at some of the node.js build configs, and saw those libraries referenced as well in certain places. @irbull do you know how we could make sure that the necessary libraries are linked ? How did you figure out that -lrt might be needed for the linux build ?