Enhance portability of va_copy()

va_copy() is a C99 feature. In C89 implementations, it's sometimes
available as __va_copy(). If not, memcpy() should do the trick.

src/load.c

@@ -12,7 +12,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <stdarg.h>
 #include <assert.h>
 
 #include <jansson.h>