Adjust library linking and provide PkgConfig support

Two changes are provided (which are somewhat dependent).

1. `libhts.so` makes use of `libm.so` and `libpthread.so`, but does not link against these libraries. When uses link against it, they must also include its dependencies, which is awkward. Linking against static library `libhts.a` for `bgzf` and `tabix` does not exhibit this problem because the linker discards all the unused object files, and so discards `kfunc.o` that needs `libm` and both include `-pthread`.

2. Pkg-Config is a popular method for linking against shared libraries. This provides the pkg-config description for consuming `libhts` using pkg-config macros in GNU AutoTools.