So that rubygems can find them as gems.  However, the `--install-dir`
option of `gem install` seems to exclude prerelease gems, even already
installed in that directory, from the dependencies for some reasons;
use the `GEM_HOME` environment variable instead.

Now net-imap 0.5.0 depends on json gem.

This caused an issue when `defined?` was in the `if` condition. Its
instructions weren't appended to the instruction sequence even though it was compiled
if a compile-time known logical short-circuit happened before the `defined?`. The catch table
entry (`defined?` compilation produces a catch table entry) was still on the iseq even though the
instructions weren't there. This caused faulty exception handling in the method.
The solution is to no add the catch table entry for `defined?` after a compile-time known logical
short circuit.

This shouldn't touch much code, it's only for cases like the following,
which can occur during debugging:

    if false && defined?(Some::CONSTANT)
    "more code..."
    end

Fixes [Bug #20501]

Single `Integer` argument means an exact match to the major version.

The instance variable @tmpfile was removed in ddcfc9f so check_tempfile_leak
did not work.

Although `pend` in test-unit gem expects the block does not pass if it
is given, our `pend` should ignore the block silently as same as it
just skips the test with no message by default.  Add an unused
parameter, to suppress the warning for the block to be ignored.