subversion17: allow build against system openssl
subversion17: allow build against system openssl

This goes against Homebrew policy, but without it, Neon can't build
with SSL support and folks can't checkout SSL/TLS secured svn repos.

Closes #790.

heuermh/homebrew-versions,UniqMartin/homebrew-versions,Ashton-W/homebrew-versions,jimhe/homebrew-versions,coeligena/homebrew-verscustomized,DomT4/homebrew-versions