Yes, this was tedious, but we do it for each of our supported platforms.
But we can't do it for various C libraries. This created a problem for us, as it is indeed tedious for users. We created a repository where people shared their conversions, but it was still inadequate.
The solution was to build a C compiler into the D compiler. Now, you can simply "import" a C .h file. It works surprisingly well. Sure, some things don't work, as C programmers cannot resist put some really crazy stuff in the .h files. The solution to that problem turned out be we discovered that the D compiler was able to create D modules from C code. Then, the user could tweak by hand the nutburger bits.
This is the same solution that Apple chose for Swift <-> Objective C interop. I wonder if someone at Apple was inspired by this decision in D!