Add an -imacros option

[tichy3000]

It would be nice to have an option like the -imacros option from cpp. (I can't see a way to achieve this behaviour with gpp)

To cite the cpp manual:

-imacros file Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. All files specified by -imacros are processed before all files specified by -include.