Compile backtrace out if unavailable on system

Compile backtrace out if unavailable on system

backtrace() is a glibc-only feature. Some systems, such as Alpine,
do not support glibc.

It seems one solution is to rely on community ports, but I imagine it'd
be best to offload such a decision to the user. Not the safest.

Instead, if backtrace() is not available, just delete stack traces from
the binary. It's going to be a pain to debug, but that's the world we
live in, I guess.

Turns libexec into an optional dependency. Fixes #87.

Also, the commit contains a review and update of the documentation's
Alpine dependency list. There was a lot of fat in there.