[Issue 21974] importC: Error: undefined identifier '__builtin_va_list'

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21974

--- Comment #7 from Walter Bright <bugzilla at digitalmars.com> ---
One theory is if a declaration can be supplied by the library or some source
file, it should *not* be built in to the compiler. This:

1. reduces the complexity of the compiler
2. makes it editable by the user
3. means updates can be done without altering the compiler
4. makes it unnecessary to get it to match every version of every C compiler
out there

We do this in D by automatically importing object.d.

I already believe we'll have to supply such a file anyway simply to deal with
all the 400 predefines that gcc generates.

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