Remove vacuous states in base64_sem?.c

I don't think these actually affect correctness since there's no way for
0x40 or 0x60 to show up in a parse tree anyway, but they're confusing.
Fix compilation when PKG_CONFIG_PATH is not in environ
Move backend names into src/benchmark.c

It's causing unreferenced-variable warnings, and isn't referenced
anywhere aside from benchmark.c. If client code is likely to reference
it, perhaps move it into another header, so people who include hammer.h
don't have to refer to it to have warning-free code.