How to install GIMP resynthesizer on Mac OS?
There are currently two relatively easy ways to get to this plugin. "Easy" as in "still a pain", just like the other, third and fourth option.


The batteries included approach, as recommended b resynthesizer: download a build from GIMP on OS X. Sadly development seems to have stopped about a year ago. Currently the latest version is 2.8.12 built for Sierra. (Original website no longer active as of May 2019. On Sourceforge the version recommended by Resynthesizer Gimp on OSX Gimp-2.8.18p1-Sierra.dmg 2017-01-08)
The 'some plugins already included approach: download a binary from Partha, Photography and Related Musings. Does not include the resynthesizer plugin itself but has recent builds against which building the plugin itself is a bit easier. Latest builds available. Currently at McGimp 2.10.8 Standard Edition (2019).
Use the help of MacPorts to build everything GIMP from scratch/source including all build-dependencies and then  go to GIMP Resynthesizer Plugin Suite and build that as well from source.
Install the official or any other binary for GIMP as usual on macOS, then install a virtual machine and a Linux in that, with GIMP and the resynthesizer plugin ready-made to play well from a package repository. Use that for the Enhance filters (if you work exclusively in the Virtual Machine you might of yourse skip the macOS part of installing GIMP)


That said, its probably easiest (and a huge hassle) to just install two versions of GIMP, like in the 4th option. A recent one without the plugin but all the latest core features, and either an older GIMP with plugin, or the GIMP on VM route.