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/signal.c

@@ -21,10 +21,10 @@ static uv_signal_t spwr;
 #endif
 
 static bool rejecting_deadly;
-static char * signal_name(int signum);
-static void deadly_signal(int signum);
-static void signal_cb(uv_signal_t *, int signum);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/signal.c.generated.h"
+#endif
 void signal_init()
 {
   uv_signal_init(uv_default_loop(), &sint);