Try to avoid many of the compiler warnings when compiling libffi by
using a proper function prototype.
diff --git a/Modules/_ctypes/libffi/include/ffi.h.in b/Modules/_ctypes/libffi/include/ffi.h.in
index 4260045..5ddda79 100644
--- a/Modules/_ctypes/libffi/include/ffi.h.in
+++ b/Modules/_ctypes/libffi/include/ffi.h.in
@@ -188,7 +188,7 @@
 } ffi_raw;
 
 void ffi_raw_call (/*@dependent@*/ ffi_cif *cif, 
-		   void (*fn)(), 
+		   void (*fn)(void), 
 		   /*@out@*/ void *rvalue, 
 		   /*@dependent@*/ ffi_raw *avalue);
 
@@ -201,7 +201,7 @@
 /* longs and doubles are followed by an empty 64-bit word.		*/
 
 void ffi_java_raw_call (/*@dependent@*/ ffi_cif *cif, 
-		        void (*fn)(), 
+		        void (*fn)(void), 
 		        /*@out@*/ void *rvalue, 
 		        /*@dependent@*/ ffi_raw *avalue);
 
@@ -270,7 +270,7 @@
 			/*@dependent@*/ ffi_type **atypes);
 
 void ffi_call(/*@dependent@*/ ffi_cif *cif, 
-	      void (*fn)(), 
+	      void (*fn)(void), 
 	      /*@out@*/ void *rvalue, 
 	      /*@dependent@*/ void **avalue);