#158 but for MacOS: pass library existence check when dynamic library exists #243
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#159 added support for using shared-library-only builds of rdkafka, but only for linux. This repeats that for MacOS. This turned out to be necessary while switching rdkafka to CMake on nixpkgs here.
Side note: i contemplated also adding windows library names, but I have no idea what the naming scheme there is, and I think these checks should better be done with some common mechanism like
find_libraryorpkg_check_modules(no idea which is proper). I btw also have never touched a MacOS, and I don't know whether this is sane, all I know is that it works on nixpkgs CI.