Implement --no-installed for gem list

This option was present but never implemented.

Fixes #593
Fixed installing gems with extensions under -V

Commit f815636f changed the return value of ExtConfBuilder#build to nil
from the input result array.  When running with -V RubyGems prints the
results of compilation by joining the returned Array.  nil does not have
this so a NoMethodError was raised.

Fixes #601
Synchronize interaction with the connection store

On JRuby this operation on the connection store hash is not thread safe
and results in race conditions.

Fixes #597
Fix extension building on Ruby 1.9.3 and mingw

Supplying DESTDIR on mingw causes link errors due to reasons I don't
understand (see the related bug).  Now, on 1.9.3, DESTDIR is not
supplied to the make command which prevents the bug.  Ruby 2.0.0 behaves
as before.

Fixes #594, see also duplicate bug #599