remove regex usage
This is a bit of an open question, wherein I want to see if others think this is a good idea.
The Graphviz tree contains exactly three usages of the POSIX regular expression API (regex.h): ./lib/gvc/gvconfig.c, ./lib/gvc/gvusershape.c, and ./cmd/smyrna/btree.c. The first two look trivial and the last one looks like it's not even used as a regex. To support these three uses, we take a dependency on RxSpencer because Windows does not support this API.
Would it be an improvement to refactor these to not use regexes at all and then drop the RxSpencer dependency?