Pass DESTDIR via command line to override what's in MAKEFLAGS.

This fixes an installation problem under a package building
environment where DESTDIR is specified in the (parent) command line.

Related bug reports:
- https://bugzilla.redhat.com/show_bug.cgi?id=921650
- https://github.com/ruby/ruby/pull/327
Merge pull request #573 from knu/destdir_fix

Pass DESTDIR via command line to override what's in MAKEFLAGS.