Can the C preprocessor be used to tell if a file exists?

Question

I have a very large codebase (read: thousands of modules) that has code shared across numerous projects that all run on different operating systems with different C++ compil

Answer 1

Create a special folder for missing headers, and make that folder to be searched last
(that is compliler specific - last item in "INCLUDES" environment variable, something like that)

Then if some header1.h can be missing, create in that folder a stub

header1.h:

#define header1_is_missing

Now you can always write

#include 
#ifdef header1_is_missing

   // there is no header1.h 

#endif