First, his so-called "prophecy" did not occur as he predicted. Second, anyone could "prophesize" something thirty years in advance, as Joseph Smith did, if they had enough advance warning via different media sources. To prove this, let me predict that in 30 years the United States and Europe will be involved in war with militant Islamic countries, gas prices will top $5.00 gallon, and a devasting earthquake will rock Los Angeles. Am I a prophet, or am I just someone who kinda-sorta follows the news and can make an educated guess?
South Carolina. Big whoop. It was well known that South Carolina had long been a state that wanted to secede from the union due to the slavery issue. In fact, can you guess which state was the first to call together its delegates to secede from the Union just after the election of Abraham Lincoln? The point is, everyone knew what was inevitably going to happen long before the Civil War broke out, and just because Joseph Smith mentions the very thing he had been reading about in his work of fiction hardly makes it a "prophecy."
Conversely, there are several things that Joseph Smith included in his Civil War prophecy that did NOT come to pass. (1) War was not poured out on all nations (D&C 87:2-3). (2) If "Great Britain" played a role in the Civil War, it was nominal at best, and hardly worth mention. (3) The slaves did not rise up against their masters (v. 4). (4) There were no such entitites as the "Gentiles" to be vexed. (5) The inhabitants of the earth did not experience famine, plague, earthquakes, thunder and lightning, nor the wrath and indignation from the hand of Almighty God, nor did the "full end of all nations" occur (v. 6). The bottom line is, that despite all the touting by the Mormons that Joseph Smith predicted the Civil War with any degree of accuracy, as some type of proof that he was a "prophet," upon closer examination, the Civil War pronouncements by him only served to prove just the opposite.