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/popupmnu.c

@@ -31,8 +31,10 @@ static int pum_col;                 // left column of pum
 
 static int pum_do_redraw = FALSE;   // do redraw anyway
 
-static int pum_set_selected(int n, int repeat);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "popupmnu.c.generated.h"
+#endif
 #define PUM_DEF_HEIGHT 10
 #define PUM_DEF_WIDTH  15