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

[d-bugmail at puremagic.com]

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

--- Comment #5 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Walter Bright from comment #3)
> What is the typedef you've added to the wrapper sources?
For testing purposes.
---
/* Provide user declarations of the built-in va_list type.  */
#if defined(__i386__)
typedef char* __builtin_va_list;
#elif defined(__x86_64__)
typedef struct
{
    unsigned gp_offset;
    unsigned fp_offset;
    void *overflow_arg_area;
    void *reg_save_area;
} __builtin_va_list[1];
#else
#error "unsupported"
#endif
---

It's not the only `__builtin_` that I've written something to provide for.
e.g:
---
#define __builtin_bswap16(x) \
    ((u_int16_t) ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8)))
#define __builtin_bswap32(x) \
    ((((x) & 0xff000000u) >> 24) | (((x) & 0x00ff0000u) >> 8) \
     | (((x) & 0x0000ff00u) << 8) | (((x) & 0x000000ffu) << 24))
#define __builtin_bswap64(x) \
    ((((x) & 0xff00000000000000ull) >> 56) \
     | (((x) & 0x00ff000000000000ull) >> 40) \
     | (((x) & 0x0000ff0000000000ull) >> 24) \
     | (((x) & 0x000000ff00000000ull) >> 8) \
     | (((x) & 0x00000000ff000000ull) << 8) \
     | (((x) & 0x0000000000ff0000ull) << 24) \
     | (((x) & 0x000000000000ff00ull) << 40) \
     | (((x) & 0x00000000000000ffull) << 56))
---

I'm currently on the fence really whether we should open the gates for handling
`__builtin_` symbols.

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