The `geany.pc` should be installed in `$prefix/lib/pkgconfig/` where it appears your prefix is default `/usr/local`. That should also be the default search for [`pkg-config`](https://linux.die.net/man/1/pkg-config) but perhaps your unknown distro has configured `pkg-config` search path differently.
If that is the case you should follow the instructions `pkg-config` gave and modify the specified environment variable to include where `geany.pc` was installed before running the build for `geany-plugins`.