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/diff.c

@@ -46,24 +46,10 @@ static int diff_flags = DIFF_FILLER;
 // checked yet
 static int diff_a_works = MAYBE;
 
-static int diff_buf_idx(buf_T *buf);
-static int diff_buf_idx_tp(buf_T *buf, tabpage_T *tp);
-static void diff_mark_adjust_tp(tabpage_T *tp, int idx, linenr_T line1,
-                                linenr_T line2, long amount,
-                                long amount_after);
-static void diff_check_unchanged(tabpage_T *tp, diff_T *dp);
-static int diff_check_sanity(tabpage_T *tp, diff_T *dp);
-static void diff_redraw(int dofold);
-static int diff_write(buf_T *buf, char_u *fname);
-static void diff_file(char_u *tmp_orig, char_u *tmp_new, char_u *tmp_diff);
-static int diff_equal_entry(diff_T *dp, int idx1, int idx2);
-static int diff_cmp(char_u *s1, char_u *s2);
-static void diff_fold_update(diff_T *dp, int skip_idx);
-static void diff_read(int idx_orig, int idx_new, char_u *fname);
-static void diff_copy_entry(diff_T *dprev, diff_T *dp, int idx_orig,
-                            int idx_new);
-static diff_T* diff_alloc_new(tabpage_T *tp, diff_T *dprev, diff_T *dp);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "diff.c.generated.h"
+#endif
 #ifndef USE_CR
 # define tag_fgets vim_fgets
 #endif  // ifndef USE_CR