Autodetect Macports installation of sqlite

[timhemel]

I have installed sqlite3 via Macports, which installs the dynamic library in /opt/local/lib/libsqlite3.dylib. On my system there is also on in /usr/lib/libsqlite3.dylib, but this looks like one that was shipped with the OS. Both locations are not searched for in clib in defs.lua, and as a result, I get the error message 'image not found'. Hardcoding it by setting vim.g.sqlite_clib_path works, but it would be nice if the autodetection would work for a Macports installed library.