Generate header file from macros

[simonbyrne]

We would need some way to specify C function signatures, but it would be nice if we could write something like

@ccallable function julia_cg(fptr::CfnPtr{Cint, (Ptr{Cdouble}, Ptr{Cdouble})}, cx::Ptr{Cdouble}, cb::Ptr{Cdouble}, len::Csize_t)::Cint
   ...
end

and generate the declaration for the header:

int julia_cg(int (*fptr)(double*,double*), double *cx, double *cb, size_t len);