Fix configuration when cross-compiling

[jmarshall]

Even when cross-compiling, build.rs runs on the build host. Hence within build.rs #[cfg(target_os)] always reflects the host, not the target. Use $CARGO_CFG_TARGET_OS instead to query target properties.

Fixes #259.

[brainstorm]

Thanks John! Could you provide some documentation and/or comments alongside as you did on twitter in the code? Also it seems like formatting is off.

[brainstorm]

Just a cargo fmt away, simples ;)

[jmarshall]

The change is described in the commit message; beyond that, I don't know what documentation you're after.

I am not a rust developer and I am unfamiliar with your expected formatting. Moreover, I am not a rust-htslib user and I am not keen to devote further time to this. I would suggest that the maintainers are best placed to make any adjustments required should they choose to merge this bug fix.

[jmarshall]

Comment added, assuming what you were after was a comment in the source itself to further dissuade anyone naively reverting this to #[cfg(target_os …)].

[coveralls]

Pull Request Test Coverage Report for Build 1032302039

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 93.543%

---

Totals
Change from base Build 1031511633:	0.008%
Covered Lines:	11503
Relevant Lines:	12297

---

💛 - Coveralls

[brainstorm]

CI seemed to be stuck picking old commit messages, even with correctly formatted with fix: <message> as indicated in https://www.conventionalcommits.org/en/v1.0.0/#summary.

Anyway, thanks John for the contribution.