macOS dylib's id does not use a fully qualified path

Xcode 12 lost the ability to test libraries linked against the C library when using the swift compiler (though ironically, swift test from the command line works just fine). After many hours of investigation, I noticed the the dylib's id is not fully qualified:
```
$ otool -L /usr/local/lib/libfdb_c.dylib
/usr/local/lib/libfdb_c.dylib:
libfdb_c.dylib (compatibility version 0.0.0, current version 0.0.0)
/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1560.0.0)
/System/Library/Frameworks/IOKit.framework/Versions/A/IOKit (compatibility version 1.0.0, current version 275.0.0)
/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.200.5)
```

Most other dylibs I compared against, and that are fine to link against do have a full path for the first line of that output (`libfdb_c.dylib (compatibility ...`). Fixing it is thankfully easy, but must be done every time the user updates FDB:

`$ install_name_tool -id /usr/local/lib/libfdb_c.dylib /usr/local/lib/libfdb_c.dylib`

Having this be part of the official release will reduce friction tremendously.

See https://github.com/kirilltitov/FDBSwift/issues/70#issuecomment-726421104 for more information.

As a second note, having a pkgconfig be installed along side the library on mac and linux would be tremendously useful, as currently, swift packages that depend on FDB must install their own each time.