Support building in Cygwin

There are a few minor things that cause a soci build to break when building under Cygwin. 

-Like FreeBSD we need to use -std=gnu++98 since snprintf isn't defined in the strict ANSI headers.
-Passing -fPIC causes a warning on every line saying that code is always position independent on this platform (not breaking, but very annoying in the output so I removed it).
-strtoll is not in the std namespace, even though it's included from <cstdlib>. To fix this, I've added a using directive of std::strtoll to non-Cygwin platforms, and changed references to std::strtoll to strtoll.

I've only gone through and fixed up the PostgreSQL and SQLite3 backends (since that's what I'm using); it's possible others are also broken.