How to most efficiently spend gold as support?
Solution 1:
For starters, 3 slots are usually taken for standard items: gold item, sightstone and boots. This leaves 3 open slots for other items. And you won't probably not fill them until 25 mins of game (unless it's unbalanced).
Most supports has been reviewed to get more utility based on their AP or an other stat. So, an efficient way to spend gold on support is by getting items that provide help in teamfight and using that particular stat.
Example (for Lulu which I play a lot), I go for Morellonomicon for CoolDown Reduction (more whimsy, more ults, more slow, more shields), then depending on the enemy team: abyssal scepter for magic resist or Zhonya's Hourglass for armour. Last item can be either twin shadows for more MR and the active or Rabbadon's Deathcap for a real boost in AP late game, since all the kit of Lulu is being more efficient with AP (stronger slow, stronger speed boost, stronger shield and more health given by the ult).
Some items remains situational (at least for Lulu) like Solari, Mickael's Crucible, Randuin or spirit visage. However, they've became more interesting for supports like Taric, which scales with armour and bonus health. This goes the same for other supports that aren't damage-oriented (like Nidalee).
By the way, I would disagree on the fact that AP builds aren't for teamfights. When utility scales on AP, then AP is good for teamfight.
I would also strongly discourage supports (non damaged oriented) to go for long-term items like rod of ages, or seraph's embrace. These need times to build up their strength, and supports need the first three items (sightstone, gold item and boots) A.S.A.P. (the first two provide gold, the last will avoid you getting constantly killed).
As a conclusion, supports can spend their gold to strengthen their advantages and diminish their weaknesses.
Solution 2:
Generally with (pre)season 4 the supports became like a full tank/ap and sometimes even ad champ. Since you can potentially compete with a mid/toplaner in gold, you can build like them. This means you usually go full tank on Leona, Taric, Alistar and so on since their skillset is based on CC and/or scaling with defensive stat while you go full ap on champs like lulu/sona/soraka. This gold boost also allowed new champs to become viable (Annie/Nidalee for example) due to the ability to build full AP and be a second mid laner. And I'm talking about a useful support Nidalee ;)
Currently you can see a huge winrate on Leona, Taric and Thresh because these champs can go full tank and still do a huge amount of damage or lock targets down with their CC. You can also build these "AP supporters" as tank but they just aren't as good on tank as you'd expect since they often scale so well with AP.
Only a few examples: Janna E: AP = bonus AD, Lulu Q/W: AP = bonus movementspeed %, bonus slow % , Sona E/Passive: AP = movementspeed %, bonus slow % and damage reduction %. And did i mention that you can easily oneshot an enemy adc with an ap support?
So generally i'd say a safe generic build would be:
For tanky supporters:
Start with shield, coin -> Boots/Sightstone -> Locket -> Randuins -> Whatever tanky item you want (Warmogs is a really good choice usually)
For AP Supporters
Start with coin/dagger(forgot the name sorry) -> Boots/Morello -> Sightstone/Zhonyias/Mikahils/Haunting guise -> Rabadons -> Random AP or tanky item (Maybe Rylais for more utility and tankiness)
For boots you can pick almost everything as long as it's not berserk. I prefer CDR on tanky supps (Leona) and utility AP supps (Janna). For ap bursters (Lulu, Zyra, Annie) i usually pick MPen boots. Of course mobility is also a good choice for roaming and helping out the other lanes + warding.