I have a VS2015 solution with a library and an executable, how can I add a preprocessor defintion to the executable that will be used by the library?

I have a 2 visual studio 2015 solutions that each contain two projects,

one is an executable, one is a library.

The library is shared (the same) between two of those solutions, the executables are different between the two solutions.

The library has some #ifdef statements in the code, like:

#ifdef project1
  //Some Code
  //More code

I want the executable project 1 to have a pre-processor definition #define project1 and project 2 to have a pre-processor definition #define project2

I run into the problem that I have to add the pre-processor definition to the library itself, instead of the executables, in order for the #ifdef directive to recognize it in the library. This is obviously not what I want

Is there a way that I can add a definition to each solution separately, and have it compile the library using that pre-processor definition, so I don't have to manually change that definition in the library each time I build one of the two solutions?

I would prefer to do this using settings in VS2015