Do fuel reserves take into account the possibility of a constant headwind?
The fuel "to reach the destination" is computed using winds, head- or tail-wind as appropriate. For "30 minutes of reserve" that is 30 minutes, and it is the same 30 minutes regardless of flying into or with the wind. In 30 minutes of flying with a tailwind you'll fly FARTHER than 30 minutes with a headwind, but 30 minutes is 30 minutes.

-- Edit to answer the question as expanded in a comment:

Wind predictions are actually pretty good now, and "30 minutes" isn't really all that the pilot has to work with.

For FAR Part 91, essentially general aviation (think taking a Cessna on a flight of a few hundred miles as an easy picture), the legal minimum in good weather is 30 minutes.  For FAR Part 121, which governs airline flying, it's 45 minutes (plus other categories as explained in the other answers for things like a destination alternate).  Beyond that, the plan for the flight will almost always include additional fuel above that to account for the routine things that happen to burn more gas: delays waiting to take off, small changes in routing that add a few miles to the planned path, and so on.  That way, even with the routine "stuff" that happens, you don't plan to eat into the 45 minutes reserve.  If the wind forecast is off, then that tends to be another of "those things" that changes the plan.

Realistically, how much are the winds off of the forecasts?  Typically, the forecasts are pretty close.  The computer models that are used now are very detailed, which is possible with all the computing power that's available now as compared to a decade or two ago.  Also, these models are constantly updated by data coming in from aircraft that are airborne.  Various datalinks can transmit the winds and temperature in real time, and ATC will also sometimes ask for current winds at a particular altitude.  All of this combines to give a picture that isn't simply a wind forecast issued in isolation, but a combination of forecast + recent data.

The issue that would arise if an airline were required to plan on the worst-case wind that might be encountered as if it would be present for the entire flight, is that in some cases the flight would significantly under-burn: it would arrive with more fuel than planned.  Why is that an issue?  Maximum Landing Weight.  If your airliner is fully loaded with passengers &amp; cargo, there is a limit to how much fuel it can have on board at landing, driven by the maximum certified landing weight.  On my airplane, that tends to be enough fuel for a bit over two hours of flying.  (Obviously a LOT of factors influence this on each flight -- this is a ballpark number.)  So if I have a full airplane and I want to plan on having as much fuel as possible upon arrival (so that we can hold if necessary, fly the approach, and still have adequate fuel for a missed approach and then a divert, arriving at the alternate with adequate margins), there is a limit to how much I can take off with.  And that limit is driven by how much fuel we're expected to burn enroute.  If my enroute burn is lower than planned and I do NOT have to hold or take a longer route than planned, I can show up too heavy to legally land.

There are two options when that happens: land anyway (safe, but requires an inspection of the aircraft, and it's generally frowned upon to do this without good reason), or fly circles to burn off the gas.  The latter is what is generally done, and is obviously wasteful.  So it's preferred that the enroute burn calculations be as accurate as possible, rather than including unrealistic headwind values.