fix building with newer glibc

http://bugs.gentoo.org/227923

--- a/pretrace.h
+++ b/pretrace.h
@@ -20,3 +20,8 @@
 #include <sys/time.h>
 # define RANDOMIZE(x) { struct timeval tv; gettimeofday(&tv, NULL); x = tv.tv_usec; }
 #endif
+
+#include <unistd.h>
+#ifndef ARG_MAX
+# define ARG_MAX sysconf(_SC_ARG_MAX)
+#endif