Static OpenSSL Linking for MacOS and Windows

[rustysec]

I made some build.rs updates to support statically linking openssl + curl on MacOS and Windows. Previously, this worked flawlessly against musl on linux, but the other platforms still required extern dependencies once compiled.

I tried to keep the changes as minimal as possible, and if there is a better way to accomplish this please just let me know.

I use curl-rust in production and have to support many platforms. Sometimes we run into old XP systems that don't support TLS natively and thus statically bringing openssl in a fat binary is the only way to achieve communications. Similarly on MacOS, I'm not able to guarantee the presence or version of a dynamically linked openssl version on the host.

I used the example app from the README and this feature set to produce binaries for windows and MacOS to verify the changes work.

Note: originally i had introduced a completely separate feature flag for forcing this as to leave the current behavior in tact. I decided against it eventually but can definitely bring it back if so desired.

[rustysec]

I'll look into the failures!

[alexcrichton]

Thanks for this! I think it might be best to pull this in as a new feature, perhaps static-openssl. That way it wouldn't break existing users of static-ssl who use it primarily only for Linux and leave native SSL support to Windows/OSX

[rustysec]

That's how I originally did it in my other branch, I was trying to make this one simpler. However, you're totally correct... I didn't quite take into consideration the static curl + dynamic openssl use case.

[rustysec]

i haven't forgotten about this! will hopefully fix up the branch soon.

[rustysec]

i'm going to close this and open another pr since my branch has drifted so far from master