Support GEOS Installation from Library

[keithdoggett]

Summary

Allows GEOS to be linked with a library (.so, .a, .dll, etc.) by specifying the RGEO_GEOS_LIB_DIR environment variable.

Other Information

This works, but there are still a couple issues that need to be discussed/resolved

• Due to how find_library works if libgeos_c is not found in the RGEO_GEOS_LIB_DIR directory and it is installed on the system (/usr/local/lib for example), it will silently fall back on the system installed version instead of raising an error.
• We may want to discuss automatically falling back on the geos-config flow if there is an error during the from library flow.
• Discuss how to test this in CI

[BuonOmo]

@keithdoggett that's nice thanks, indeed the fallback may be really missleading if someone is expecting the build from a specific library.

Also, and more importantly, I'm not sure where we want to go with this. E.g. in which circumstances someone would want to load geos from somewhere else? And in that case, why not loading geos from somewhere in the path of this library (#272).

Hence I think this is a nice step towards getting in that place, since we'll only have to specify GEOS's local path then, but I just want to be sure about the reason why you started from that end :)

[keithdoggett]

@BuonOmo yeah the main reason was to make that packaging easier eventually. I guess I figured that this was the hard part since I hadn't seen a C extension built in this way before, but there are some other use cases.

1. Some programs ship with GEOS and if you're using that library for some part of your system it may make sense to ensure that RGeo has the same version so you're not running different versions of GEOS. (one example: https://www.cockroachlabs.com/docs/stable/install-cockroachdb-linux.html).

2. Right now the only way to install it is if geos-config is installed on your system. In some environments it may be easier to work with the compiled libraries directly, especially if you want to use a version that is not available on the package manager by default. This just adds some more flexibility.

[BuonOmo]

I definitely agree with you

I guess I figured that this was the hard part since I hadn't seen a C extension built in this way before

I've seen some, for instance the ruby-sass!