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/log.h

@@ -10,9 +10,6 @@
 #define WARNING_LOG_LEVEL 2
 #define ERROR_LOG_LEVEL 3
 
-bool do_log(int log_level, const char *func_name, int line_num,
-            const char* fmt, ...) FUNC_ATTR_UNUSED;
-
 #define DLOG(...)
 #define ILOG(...)
 #define WLOG(...)
@@ -53,5 +50,7 @@ bool do_log(int log_level, const char *func_name, int line_num,
 
 #endif
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "log.h.generated.h"
+#endif
 #endif  // NVIM_LOG_H
-