build: update htslib & fix build.rs

[pmarks]

@brainstorm this fixes build.rs on top of #237. Fix works on Mac, let's see what happens on other platforms. This at least keeps us going with the current style of build.rs

[coveralls]

Pull Request Test Coverage Report for Build 1038756060

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.008%) to 93.535%

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Files with Coverage Reduction	New Missed Lines	%
src/bam/record.rs	1	74.21%

Totals
Change from base Build 1032305619:	-0.008%
Covered Lines:	11502
Relevant Lines:	12297

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💛 - Coveralls

[brainstorm]

Thanks @pmarks !