feature request: #ifdef preprocessing in scc1

Nobody has claimed this yet.

The way the d directory is currently built, some .d sources depends on flags (e.g. XML, Python, or MySQL). However, if scc1 supported #ifdefs in D syntax, then we could put the list in a file and build everything in it, knowing that the compiler would skip the content if a flag is not set.

> The way to do that is this: make a shell script called `c/scc`. It would run `cpp` on the file first and then run `c/scc1`. The code of `scc1` would be modified to pay attention to the lines left behind `cpp` : a line such as `# 11 "foo.d" 2` would tell it that the next line (check that) is line 11 from the file "foo.d". We need that information for giving proper line numbers in error messages, and we don't want to be off even by 1.

_Originally posted by @DanGrayson in https://github.com/Macaulay2/M2/pull/1190_

Start with the existing c/scc1 entry point and the proposed c/scc shell wrapper, then inspect how cpp emits lines such as # 11 "foo.d" 2. Done means preprocessing can skip conditionally disabled D content while scc1 preserves the original file names and line numbers in error messages.

Written by the indexing model from the issue text.