configure for FileGDB: add explicit linking to libfgdbunixrtl (requires FileGDB SDK >= 1.2) (#5215); also try .dylib extension (#5221)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26459 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx