Remove support for building with LLVM 2.8. Fixes issue #66.

Both 2.9 and top-of-tree generate substantially better code than
LLVM 2.8 did, so it's not worth fixing the 2.8 build.

expr.cpp

@@ -2569,7 +2569,7 @@ ExprList::GetConstant(const Type *type) const {
     }
 
     if (dynamic_cast<const StructType *>(type) != NULL) {
-#if defined(LLVM_2_8) || defined(LLVM_2_9)
+#if defined(LLVM_2_9)
         return llvm::ConstantStruct::get(*g->ctx, cv, false);
 #else
         LLVM_TYPE_CONST llvm::StructType *llvmStructType =