What would you build if you were determined to play a more support-oriented role in a group with a bunch of full-casters? And not in a "be the meat-shield/damage-dealer" type of way.
My first thought was a Court Poet Skald, to buff up their INT/CHA. WIS casters are out of luck, but it covers 2 out of the 3 casting stats so still pretty good. Are there any other classes that are particularly good at boosting up casters?
Court Poet is really good, but there are other options too. One thing you could do is specialize in summons and use them to fill the meat shield role. But thats probably not what you are interested in.
Other options include building a Witch or Mesmersist who specializes on demoralizing targets. In addition to Evil Eye/Hypnotic Stare punishing Will saves by -2, shaken also punishes saves by another -2 making for quite a potent way to augment save/die spells. Of course, you could make your Court Poet Skald do demoralization too.
The reason I suggest Mesmersist is for the tricks and the Touch Treatment effect, allowing you to help your casters through dealing with conditions similar to being a paladin along with some other goodies too.
Notable, a Fox Shape Kitsune Vizier is quite a fun little build as you appear to be nothing more than a fox.
There are teamwork feats which are handy for spellcasters, but spellcasters rarely take them. If you have those feats and a means of granting them that's a useful trick. An bard, a skald whose performance doesn't interfere with spellcasting or especially an inquisitor (any of whom can cast the coordinated effort spell) might be good for this.
Collaborative thaumaturgy is a spell which might be useful, but just about any non-psychic caster can get it.
Magician bards can directly buff spellcasters with their bardic performance. Stealing a few spells off other spell lists is handy too.
In terms of teamwork feats for spellcasters, maybe take a look at Allied Spellcaster, Callous Casting, Elemental Commixture, or Escape Route.
Alternatively, how about a dual-cursed life oracle? Immediate action misfortune lets you make sure enemies tank their saves while your allies make their own, life link is great for shoring up low caster hp, and magic variant channeling lets you boost everyone's concentration and caster level checks in a pinch.
Elemental commixture is fun. Besides the others Avoron mentioned consider coordinated blast.
Combat feats with minimal prereqs useful to most spellcasters. Um. There are spellcasters who'd definitely like point blank and precise shot but never got around to getting them. Defensive combat training's usefulness depends on how much your GM likes combat maneuvers, but basically any spellcaster benefits from it. If they're willing to work with you and take some skills then improved unarmed strike and snake style could be useful.
PF Unchained's variant multiclass (witch) would give you hexes. They're still based off Int and you don't get many or soon, but it is possible.
A magician bard gets the Dweomercraft performance in place of Inspire Courage, which can be pretty nice for a group of full casters.
Dweomercraft (Su): A magician can use performance to manipulate magical energies. Allies of the magician gain a +1 bonus on caster level checks, concentration checks, and attack rolls with spells and spell-like abilities. This bonus increases by +1 at 5th level and every six levels thereafter. This ability relies on visual and audible components. It replaces inspire courage.
They also do fairly well at counterspelling, between Improved Counterspell as a bonus feat and the Spell Suppression performance. At higher levels, they can get a lot of use out of wands.
While it takes some levels to flesh out... a blood rager can make a fantastic support build. The goal is to combine an arcane strike build with a corungon smash/hurtful intimidation build.
You will need to take blooded arcane strike feat in order to remove its swift action (since the two sets of feats both use that).
But the goal is to combine three debuffs:
1. shaken- -2 to a lot of d20 rolls, including saves.
2. riving strike- -2 on saves vs. spells when you hit something with arcane strike.
3. Cruel weapon- if the target is shaken, hitting it again makes it sickened... for another -2 on d20 stuff like saves. Since a hurtful build is designed for this kind of set up... free debuff.
Combined, you can force enemies to take a -6 to saves, and you can do that with a move+attack. Any party with casters would love this. Just move from enemy to enemy, debuffing them and setting them up for the casters. You can also get in some decent damage while doing so. I like to grab a hammer and pretend I am knocking out the part of their memory that covers how to make saves.
Combat feats with minimal prereqs useful to most spellcasters. Um.
Don't forget about Amateur Swashbuckler for dodging panache! Everyone in your party will qualify, and it can be a lifesaver for staying out of melee range, especially if someone would get hit by a full-attack after a 5-ft. step or a pounce.
For ranged attacks it might be worth it to hand out Improved Unarmed Strike and Deflect Arrows, or better yet Shield Focus and Missile Shield if everyone's got at least a mithral buckler.
In terms of actual rage powers, it might not be a terrible idea to grab the Energy Resistance or Elemental Blood line to let your blasters go crazy and maybe give everyone a movement boost.
Right, Riving Strike is huge because its another -2 to folks saves. Additionally, there are probably some pretty cool spells out there that also focus purely on debuffing.
Come to think of it, you could make a Debuffing Hexcrafter Magus who specializes in Disarm, Evil Eye, and using Spellstrike to deliver curses like Bestow Curse and other sick debuffs.