Avoid conditional use of #define in header files. This might make it more difficult for the linker to eliminate duplicate information.

Modify your C or C++ source files so that header files are #included in the same order.

Partition header information into smaller blocks. That is, use a larger number of smaller header files rather than a smaller number of larger header files. This helps the linker to eliminate more of the common blocks.

Only include a header file in a C or C++ source file if it is really required.

Guard against the multiple inclusion of header files. Place multiple-inclusion guards inside the header file, rather than around the #include statement. For example, if you have a header file foo.h, add:

#ifndef foo_h
#define foo_h

...
// rest of header file as before
...
#endif /* foo_h */

You can use the compiler option --remarks to warn about unguarded header files.

Compile your code with the --no_debug_macros command-line option to discard preprocessor macro definitions from debug tables.

Consider using (or not using) --remove_unneeded_entities.