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_c.h

@@ -1,12 +1,9 @@
 #ifndef NVIM_INDENT_C_H
 #define NVIM_INDENT_C_H
+
 #include "nvim/vim.h"
-int cin_islabel(void);
-int cin_iscase(char_u *s, int strict);
-int cin_isscopedecl(char_u *s);
-int cin_is_cinword(char_u *line);
-int get_c_indent(void);
-void do_c_expr_indent(void);
-void parse_cino(buf_T *buf);
-pos_T * find_start_comment(int ind_maxcomment);
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "indent_c.h.generated.h"
 #endif
+#endif  // NVIM_INDENT_C_H