htslib 1.20 fails to build on Mac OS X 10.7.5 with the version of Apple's fork of the clang compiler (Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)) that comes with Xcode 4.6.3 (the last version compatible with Mac OS X 10.7) due to:

Undefined symbols for architecture x86_64:
  "___cpuid_count", referenced from:
      _htscodecs_tls_cpu_init in rANS_static4x16pr.o
  "___get_cpuid_max", referenced from:
      _htscodecs_tls_cpu_init in rANS_static4x16pr.o
ld: symbol(s) not found for architecture x86_64

As far as I know, versions of Apple clang less than 503 have this deficiency. Using a newer version of clang is a workaround—either a newer version of Apple clang on newer versions of macOS, or a newer version of llvm.org clang installed manually.