Linker options for Mac

[小泉 悟]

Dear Liberty Eiffel community

Thank you for merging my patches.
Now I can compile my Eiffel programs on my Mac.
However, I still can't compile a program that uses the SQLITE_DATABASE class.
If you add the following two lines to 
src/wrappers/database/library/sqlite/externals/generated/plugin/c/linker_options,
such a program can be compiled.

[UNIX.Darwin]
sqlite: "$(pkg-config --libs sqlite3)"

I have confirmed that this change does not affect FreeBSD, but I am not sure 
that this is the correct fix.
Is there a better way to do this?

KOIZUMI Satoru