makeparallel: unset MAKEFLAGS and MAKELEVEL

makeparallel inherits values for MAKEFLAGS and MAKELEVEL from make
through the environment, but they should not be propagated to the child
process in case the child process tries to run make again.

Change-Id: I4c5df10ea8055cd1f1f61a892d5b1a7acb287bbb

tools/makeparallel/makeparallel.cpp

@@ -341,6 +341,8 @@ int main(int argc, char* argv[]) {
     error(errno, errno, "fork failed");
   } else if (pid == 0) {
     // child
+    unsetenv("MAKEFLAGS");
+    unsetenv("MAKELEVEL");
     int ret = execvp(path, args.data());
     if (ret < 0) {
       error(errno, errno, "exec %s failed", path);