/SEPARATE_COMPILATION D=/NOSEPARATE_COMPILATION

 /[NO]SEPARATE_COMPILATION

 Controls whether an individual compilation unit becomes a separate
 module in an object file.

 The default is /NOSEPARATE_COMPILATION, which groups individual
 compilation units as a single module in an object file.

 When creating modules for use in an object library, consider using
 /SEPARATE_COMPILATION to minimize the size of the routines included
 by the linker as it creates the executable image.  In most cases,
 the default setting (/NOSEPARATE_COMPILATION) allows more
 interprocedural optimizations.