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/indent.h

@@ -1,14 +1,9 @@
 #ifndef NVIM_INDENT_H
 #define NVIM_INDENT_H
+
 #include "nvim/vim.h"
-int get_indent(void);
-int get_indent_lnum(linenr_T lnum);
-int get_indent_buf(buf_T *buf, linenr_T lnum);
-int get_indent_str(char_u *ptr, int ts);
-int set_indent(int size, int flags);
-int copy_indent(int size, char_u *src);
-int get_number_indent(linenr_T lnum);
-int inindent(int extra);
-int get_expr_indent(void);
-int get_lisp_indent(void);
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "indent.h.generated.h"
+#endif
 #endif  // NVIM_INDENT_H