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/os/time.h

@@ -4,15 +4,13 @@
 #include <stdint.h>
 #include <stdbool.h>
 
-void time_init(void);
 
-void os_delay(uint64_t milliseconds, bool ignoreinput);
 
-void os_microdelay(uint64_t microseconds, bool ignoreinput);
 
-struct tm *os_localtime_r(const time_t *clock, struct tm *result);
 
-struct tm *os_get_localtime(struct tm *result);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/time.h.generated.h"
+#endif
 #endif  // NVIM_OS_TIME_H