Is importC ready?

[bachmeier]

I tried to compile a simple C file this morning, containing

```
#include "nmath.h"

double fsign(double x, double y)
{
#ifdef IEEE_754
     if (ISNAN(x) || ISNAN(y))
	return x + y;
#endif
     return ((y >= 0) ? fabs(x) : -fabs(x));
}
```

I assumed this would be straightforward since it doesn't have a 
bunch of dependencies and it's only a few lines of code. Instead, 
I got a stream of errors. After running the file through the 
preprocessor, I had to comment out anything with

-  `__extension__`
- Restricted pointers `*__restrict`
- `_Float128`
- `__asm__`

And in the source, anything with ISNAN, or in the preprocessed 
file,

- `__builtin_isnan`

I [filed a bug](https://issues.dlang.org/show_bug.cgi?id=22496). 
After this experience, it's not obvious to me that importC is 
ready to be part of a stable compiler release. Maybe I just had 
bad luck with the choice of file.