ImportC and druntime's importc.h and __builtins.di

[Walter Bright]

Lots of people are trying out ImportC, which is good news! Lots of people 
encounter a problem here and there with a strange error message emanating from 
the compiler when compiling a .h file.

A good way to find out what is causing this error message is to run the C 
preprocessor (such as cpp) over the .c file that #include's it, and direct the 
result into a .i file.

A bit more work, and usually the offending line in the .i file is isolated.

A large number of these errors result from the .h file making use of some 
bizzaro C language extension, that nobody who makes these .h files seems to be 
able to resist.

It is not practical for ImportC to implement every C compiler's nutburger 
extension. But usually there is a way to write a C macro to provide a workaround.

This is the job of druntime/src/importc.h, which is automatically #include'd by 
ImportC:

https://github.com/dlang/dmd/blob/master/druntime/src/importc.h

To fix the problem you've encountered, adding a macro to importc.h can get your 
code moving again (and please file a bugzilla or a PR so everyone can take 
advantage of your findings!).

Another file used to compensate for nutburger C compiler extensions is 
__builtins.di:

https://github.com/dlang/dmd/blob/master/druntime/src/__builtins.di

which, having access to D's constructions, can often fake a C compiler 
extension. __builtins.di is also automatically imported by every compilation of 
an ImportC C file.