[Issue 23401] importc can't select preprocessor

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23401

--- Comment #3 from Adam D. Ruppe <destructionator at gmail.com> ---
I'd do a command line switch.

`-importc-cpp=whatever`

sets whatever as the command to use as the preprocessor.

Then copy gdc's

`-Xpreprocessor arg`

to pass extra arguments to the preprocessor.

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