Clean up no-op AC_SUBST calls

The following discussion from !985 (merged) should be addressed:

@michal started a discussion: (+1 comment)
It seems that we can drop:
- STD_CDEFINES
- STD_CINCLUDES
- STD_CWARNINGS
That would require updating OPTIONS. There are also lots of other AC_SUBST calls for unused macros, so maybe let's tackle this in a separate issue?

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