yat.pc should use htslib.pc

[Peter]

When building in htslib mode, the yat.pc looks like this:

Requires:  zlib
Requires.private: gsl > 1.8 zlib
Libs: -L${libdir} -lyat  -lhts -lhts -lboost_thread -lboost_system  -L/usr/lib64  
Libs.private:  -lhts -lhts -lboost_thread -lboost_system
Cflags:  -pthread  -isystem /usr/local/include/htslib-1  -I${includedir}

A number of errors: 1) Requires and Requires refer to zlib 2) Libs and Libs.private are both referring to -lhts -lhts [x2]

libhts provides a pc file, so that should be referred to in Require.private and then there is no need to refer to zlib as that requirement comes from the bam support.

[Peter]

(In [3443]) fixes #850