Having failed to build 3.5.12 on FreeBSD 10.2Stable, I reverted the Makefile to 3.5.11  
svnlite update -r '{2015-12-02}' /usr/ports/www/squid/Makefile
to ensure that everything was working. I then built the port on both amd64 and i386. :)

Ok lets try 3.5.12 again:
#svnlite update /usr/ports/www/squid/Makefile
Updated to revision 404052.
# cd /usr/ports/www/squid 
# make showconfig |grep =on
     AUTH_LDAP=on: Install LDAP authentication helpers
     AUTH_SASL=on: Install SASL authentication helpers
     AUTH_SMB=on: Install SMB auth. helpers (req. Samba)
     DOCS=on: Build and/or install documentation
     EXAMPLES=on: Build and/or install examples
     FS_AUFS=on: AUFS (threaded-io) support
     FS_DISKD=on: DISKD storage engine controlled by separate service
     IPV6=on: IPv6 protocol support
     KQUEUE=on: Kqueue(2) support
     SSL=on: SSL gatewaying support
     SSL_CRTD=on: Use ssl_crtd to handle SSL cert requests
     GSSAPI_HEIMDAL=on: Build with Kerberos support from security/heimdal

# make makesum; make clean; make -DBATCH package
fails with 
===>  Script "configure" failed unexpectedly.
Please report the problem to timp87@gmail.com [maintainer] and attach the
"/var/ports/usr/ports/www/squid/work/squid-3.5.12/config.log" including the
output of the failure of your make command. Also, it might be a good idea to
provide an overview of all packages installed on your system (e.g. a
/usr/local/sbin/pkg-static info -g -Ea).
*** Error code 1

# egrep "mit-krb5-gssapi" "/var/ports/usr/ports/www/squid/work/squid-3.5.12/config.log"
configure:23572: $PKG_CONFIG --exists --print-errors "mit-krb5-gssapi mit-krb5"
Package mit-krb5-gssapi was not found in the pkg-config search path.
Perhaps you should add the directory containing `mit-krb5-gssapi.pc'
Package 'mit-krb5-gssapi', required by 'world', not found

# make -VHEIMDAL_HOME -VGSSAPIBASEDIR
/usr/local
/usr/local

This is my third and fourth attempt to build.  I use ccache, but cleaned and reset it; I also rm all work files and did a complete pkg delete -fa before this current cycle.

Has anyone built squid 3.5.12 on FreeBSD 10.2Stable using heimdal?  Unfortunately searching the squid lists, this might be relevant:

From http://lists.squid-cache.org/pipermail/squid-announce/2015-November/000033.html
"* Bug #4228: links with krb5 libs despite --without options

The Kerberos library --without-mit-kb5 and --without-heimdal-krb5
options were not working in previous 3.5 releases and could result in
build errors. This has been corrected."

Unfortunately I migrated from 3.4 to 3.5 on this build iteration, however 3.5.11 appears to have the required libraries
/usr/local/sbin/squid:
        librt.so.1 => /usr/lib/librt.so.1 (0x28521000)
        libmd5.so.0 => /usr/local/lib/libmd5.so.0 (0x28527000)
        libssl.so.8 => /usr/local/lib/libssl.so.8 (0x28529000)
        libheimntlm.so.0 => /usr/local/lib/heimdal/libheimntlm.so.0 (0x28584000)
        libhx509.so.5 => /usr/local/lib/heimdal/libhx509.so.5 (0x2858a000)
        libcom_err.so.1 => /usr/local/lib/heimdal/libcom_err.so.1 (0x285c9000)
        libcrypto.so.8 => /usr/local/lib/libcrypto.so.8 (0x285cc000)
        libasn1.so.8 => /usr/local/lib/heimdal/libasn1.so.8 (0x2875d000)
        libwind.so.0 => /usr/local/lib/heimdal/libwind.so.0 (0x287f1000)
        libroken.so.18 => /usr/local/lib/heimdal/libroken.so.18 (0x28819000)
        libcrypt.so.5 => /lib/libcrypt.so.5 (0x28828000)
        libm.so.5 => /lib/libm.so.5 (0x2884c000)
        libkrb5.so.26 => /usr/local/lib/heimdal/libkrb5.so.26 (0x28876000)
        libgssapi.so.3 => /usr/local/lib/heimdal/libgssapi.so.3 (0x288e0000)
        libltdl.so.7 => /usr/local/lib/libltdl.so.7 (0x28912000)
        libc++.so.1 => /usr/lib/libc++.so.1 (0x2891b000)
        libcxxrt.so.1 => /lib/libcxxrt.so.1 (0x289ca000)
        libgcc_s.so.1 => /usr/local/lib/gcc48/libgcc_s.so.1 (0x289e4000)
        libthr.so.3 => /lib/libthr.so.3 (0x289ff000)
        libc.so.7 => /lib/libc.so.7 (0x28a20000)
        libz.so.6 => /lib/libz.so.6 (0x28b92000)
        libheimbase.so.1 => /usr/local/lib/heimdal/libheimbase.so.1 (0x28ba7000)
        libintl.so.8 => /usr/local/lib/libintl.so.8 (0x28bab000)

And finally
# uname -aKU
FreeBSD b2.hs 10.2-STABLE FreeBSD 10.2-STABLE #0 r292253M: Wed Dec 16 01:06:47 AEDT 2015     root@hathor:/usr/obj/prod/100202/D/K8/pd3/src/sys/hqdev-amd64-smp-vga       amd64 1002503 1002503

Happy to assist further.
Regards, Dewayne.