In generators such as Ninja that can run multiple "cl" processes that
refer to the same compiler .pdb file (/Fd) at the same time, MSVC from
Visual Studio 2013 complains:

 fatal error C1041: cannot open program database '.../vc120.pdb';
 if multiple CL.EXE write to the same .PDB file, please use /FS

According to "cl /?":

 /FS force to use MSPDBSRV.EXE

Add the flag to compilation lines for this compiler version just after the
/Fd option.