Work around lack of execvpe on FreeBSD

Commit 06468ef switched `test-ptrace.c`
to use `execvpe` instead of `execve`. However, the former doesn't exist
on FreeBSD. This now adds a guard such that `execve` is used on FreeBSD
and `execvpe` is retained on other systems.

tests/test-ptrace.c

@@ -233,7 +233,11 @@ main (int argc, char **argv)
         fprintf(stderr, "Need to specify a command line for the child\n");
         exit (-1);
       }
+#ifdef __FreeBSD__
+      execve (argv[optind], argv + optind, environ);
+#else
       execvpe (argv[optind], argv + optind, environ);
+#endif
       _exit (-1);
     }
   atexit (target_pid_kill);