Add a SYS_write definition on systems where it is not defined in headers

Namely, in the Android NDK headers, SYS_write is not defined; but
__NR_write is.

include/jemalloc/internal/jemalloc_internal.h.in

@@ -1,6 +1,9 @@
 #include <sys/mman.h>
 #include <sys/param.h>
 #include <sys/syscall.h>
+#if !defined(SYS_write) && defined(__NR_write)
+#define	SYS_write __NR_write
+#endif
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/uio.h>