Refactoring duplicate code and logic in prep for DESTDIR deprecation.

DESTDIR is slated to be deprecated. The block of code that checks for
DESTDIR contained duplicate code as the block that checks for --destdir.

The dupe code has been moved out of the destdir checks.

I have also flipped the order of checking. Previously, if the DESTDIR env was set
it would be used regardless of whether or not the --destdir flag was set.

No env, no flag:
./install.rb
destdir = nil

Env only:
DESTDIR="foo" ./install.rb
destdir = foo

Flag only:
./install.rb --destdir="bar"
destdir = bar

Both (uses flag):
DESTDIR="foo" ./install.rb --destdir="bar"
destdir = bar