compat: Add timegm() function

Windows actually has a _mkgmtime() function, but it causes link failures
that I didn't feel like debugging.  So let's just copy a generic
implementation from Android (Bionic) and then it will work on other
platforms too.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
sdtid: Use timegm() instead of mktime() for date parsing

mktime() adjusts the result based on the local machine's timezone offset,
which is not what we want when translating dates.

fixes #19

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Tweak AC_CHECK_FUNCS quoting

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
travis: Run "make check" in several different timezones

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Bump version to 0.81 and update changelog

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>