Revert "PERL-202 lame hack to replace non-portable timegm()"

This reverts commit cab04755b5f79d42af0150c9eeddb538ba4ff9b1,
which replaces a library function not available on Win32 (timegm)
with a different library function not available on Win32 (setenv).

(Some days, you just can't win.)
Allow fsync.t to pass on systems that do not support async flush

Some ports of MongoDB (i.e. Windows) do not support asynchronous flushing, causing the async flush tests to fail.
Detect this scenario and skip them.
Some tests are using old obsolete AUTOLOAD magic.  Update them to use get_database() to prevent warnings from being generated.
Change "warn on floating timezone" test into a genuine test and suppress warning output.
Suppress warnings caused by strange int() calls performed to force internal bits set on SVs.
Merge pull request #53 from Stevie-O/bug202fix

Fix for PERL-202