undefined reference to ''gattlib_connect'
This sounds like a compile-time error, not a link error. You have probably not added the proper #include to your source files for the headers for your library. It isn't enough just to add the header name to the .pro file.

You will probably end up with link errors as well, because your LIBS doesn't contain the name of the library to link to; you have only specified the directory it lives in.