Expanding C-code ifdef/ifndefs-endif-groups only.

[Per Nordlöw]

We have some bad C code that we need to straighten out by forking
various files into target-specific files by running the code through a
preprocessor that expands only certain ifdefs/ifndefs-endif-groups that
are related to specific variables such VXWORKS, etc. We do not want the
preprocessing to expand normal macros.

How can this be done, preferably in Emacs, or in some other GNU tool?

I have glanced at the manual page for cpp but it seems to expand
everything by default. Can we flag cpp to expand certain
ifdef/ifndefs-endif-groups only?

Thanks in advance,
Per Nordlöw