check TMPDIR before incremental compilation
#!/bin/sh -e
-if test -z "$TRAVIS" && rustc -Zhelp | egrep '\<incremental=' > /dev/null; then
+if test -z "$TRAVIS" && test -n "$TMPDIR" && rustc -Zhelp | egrep '\<incremental=' > /dev/null; then
export RUSTFLAGS="$RUSTFLAGS -Zincremental=$TMPDIR/grpc-rust"
fi
