Darwin: Search for both shared and convenience libraries in each path. (#761) For several Darwin linker versions, the default behaviour is to first traverse the set of library paths, looking for shared libraries, and then a second time looking for archives. This means that if the library is configured --disable-shared the installed system version /usr/lib/libffi.dylib will be found before the archive under test. Using the linker option '-search-paths-first' alters the strategy to search each path for dylib and then archive before moving on to the next.