THe C language has a very well defined stack frame and calling convention for it's functions. It has been stable for many years, and is probably the most widely used interface in computing today - Windows API calls, and Unix/Linux System calls use the C calling convention. Many (most?) languages can build a function that has the C stack frame, and call fucntions using the C stack frame. In most cases, this requires the compiler to be told not to use it's native calling convention.
The "allow exporting...." statement is a way of saying that if the language can make and/or use a C style stack frame, it will work.