Enable tzset when x-compiling to linux target

[bcmyers]

When cross-compiling, configure cannot detect if the target system has a working tzset function in C. This influences whether or not the compiled python will end up with the time.tzset function, which is used by django and other important python projects.

With more and more folks trying to move to arm64 machines in the cloud (since they are often cheaper), it would be nice to have access to time.tzset on aarch64-unknown-linux-gnu.

Since all linux targets should have a working tzset function in C, we should be able to just manually indicate this to the configure script. I've created a PR (#195) that does this.

[indygreg]

Closes by #195.