Okay I'm definitely trying to use `importC` rather than the
regular `extern(C)` way. I can see where I went wrong. The steps
for importC for header files are:
1. Prepend the following directives to `test_og.c` or whatever
your interface c script is:
```
#define __restrict restrict
#define __asm__ asm
#define __extension__
#define __inline
#define __builtin_bswap16
#define __builtin_bswap32
#define __builtin_bswap64
```
2. Run the commands:
```
gcc -E -P test_og.c > test_c.c
dmd test.d test_c.c && ./test
```
which works. Now I've tried the same thing with library `fftw3`
and getting:
```
Error: undefined identifier `__float128`
```
Which I guess I have to define somehow in the original c
directives?
