This is preprocessor option, of course.

Without this define
DllDef is defined as:
__declspec( dllexport ) - for DLL build
and as
__declspec( dllimport ) - for your app build.

So, it works as expected: DLL exports symbols and your app imports it.

If you define LIBRAW_NODLL (for both library build and for your app build)
DllDef is defined as empty string.
So, in library this is 'just' usual functions (without extra dllexport attributes)
And your app expects just usual functions, not dllimport