Fix DESTDIR related 'make' issue when DESTDIR specified

`make` has an order which applies environment variable.
If we disable existence environment in Makefile,
then using make's argument is better.

I hit this issue when try to make a RPM package with Ruby 2.0 (this report is related: https://bugzilla.redhat.com/show_bug.cgi?id=921650 ).
I don't know the better solution but this bug is critical for our RPM packaging.

And maybe, 2.1 has same bug.