IPv6 option

Hi, all: 

In the "configure" file of MozLDAP distribution, I found the option to enable IPv6: 
=============================================================
# Check whether --enable-ipv6 or --disable-ipv6 was given.
if test "${enable_ipv6+set}" = set; then
  enableval="$enable_ipv6"
   if test "$enableval" = "yes"; then
            USE_IPV6=1
      fi
fi
=============================================================
I understand that if the source is configured like "./configure ... --enable-ipv6=yes", then USE_IPV6 would be set to 1.

But where should this flag be used?
=============================================================
ifdef USE_AUTOCONF
...
ifeq ($(USE_IPV6),1)
DEFINES += -D_PR_INET6
endif

else # ! USE_AUTOCONF
...
####################################################################
#
# The NSPR-specific configuration
#
####################################################################
...
ifeq ($(USE_IPV6),1)
OS_CFLAGS += -D_PR_INET6
endif
...
endif # USE_AUTOCONF
=============================================================
This is the only place the flag USE_IPV6 is used. I gather that it will add -D_PR_INET6 to DEFINES or OS_CFLAGS. But I can't seek any futher beyond this. In the source code, I didn't find the location of -D_PR_INET6 at all. How this flag should be used?

I am quite confused now...

Looking forward to help,
Xu Qiang