Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/os/input.c

@@ -25,12 +25,11 @@ typedef enum {
 static RStream *read_stream;
 static bool eof = false, started_reading = false;
 
-static InbufPollResult inbuf_poll(int32_t ms);
-static void stderr_switch(void);
-static void read_cb(RStream *rstream, void *data, bool eof);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/input.c.generated.h"
+#endif
 // Helper function used to push bytes from the 'event' key sequence partially
 // between calls to os_inchar when maxlen < 3
-static int push_event_key(uint8_t *buf, int maxlen);
 
 void input_init()
 {