#1618 Provide complete C prototypes for C-exported functions

[Colomban Wendling]

A C function declaration with an empty parameter list does not declare a function taking no parameters, but simply does not declares what parameters it takes (hence, it's not a complete prototype). To denote an empty parameter list, C uses the special void value.

This avoids warnings when building C sources including Scintilla.h if using e.g. GCC's -Wstrict-prototypes option, as well as actually disallowing passing parameters to those functions.

[Neil Hodgson]

Committed as [4b9388].