6.6.7.15 -imacros file

Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from the file is discarded, the only effect of -imacros file is to make the macros defined in file available for use in the main input.

Any -D and -U options on the command line are always processed before -imacros file, regardless of the order in which they are written. All the -include and
 -imacros options are processed in the order in which they are written.