Allow static linking

[astraw]

This will allow building the april tags library from source into a statically linked library. There are two advantages for my use case. First, there is no need to distribute and install the april tags dynamic library. Second, the cmake binary does not need to be installed at build time.

[jerry73204]

Please take a look at this preview. Thanks.

[jerry73204]

It's nice to have the static linking feature. Furthermore, I think we should introduce another environment variable to hint the build method instead of CMAKE= and RAW= prefix. Think of an edge case that the directory name happens to be RAW=, though it would rarely happen, but it's for the sake of correctness.

Besides, I open an issue to document these magic env vars. I'll take care of it once it's merged.

[astraw]

I updated the PR by creating the APRILTAG_SYS_METHOD env var which specifies the method used to compile and link the apriltag C library. I think this resolves your points above. I also started documenting the env vars in README.md.

[jerry73204]

Great! Let's merge.

Please click the merge button straight to merge with your name.

[astraw]

Done, thanks for your review.

I have a couple other suggestions I will make PRs for in the next few days. After that, it would be great to make a new release, which I am happy to do.