I've "inherited" a couple of scripts (from someone I can't ask any more) to build Subversion on Windows. It first builds the dependencies (APR, APR-ICONV, APR-UTIL, OPENSSL, ...) and then starts the Subversion build. I reckon, by default it only builds with support for IPv4. The APR/README file states:
... The most common change is
#define APR_HAVE_IPV6 1
rather than 0 if this build of APR will be used strictly on machines
with the IPv6 adapter support installed.
...
which sounds to me like changing this option to 1 will only support IPv6. But how to build Subversion to support IPv4 and IPv6?
Copying Turbo J's answer here to be able to accept it:
Turning on IPv6 support will not affect IPv4 support - it will use a newer set of operating system functions that support both protocols. These may not work on windows versions older than Vista, and XP had an experimental IPv6 support that could be installed.