build: Fix randomly missing -I entries

The algorithm for building the consensus CFLAGS was flawed in that the
special-casing of `-I path` options only succeeded if the given option
was in the first entry of the compile commands list.

While the algorithm can certainly need an overhaul, this is the
minimally intrusive fix.

build.rs

@@ -102,6 +102,15 @@ fn main() {
                             keep
                         })
                         .collect();
+                    // Hot-fixing the merging algorithm to even work when an (always to be kept) -I
+                    // is not in the initial set
+                    for group in cflag_groups.drain(..) {
+                        if group[0].starts_with("-I") {
+                            if !consensus_cflag_groups.contains(&group) {
+                                consensus_cflag_groups.push(group);
+                            }
+                        }
+                    }
                 }
             } else {
                 consensus_cflag_groups = Some(cflag_groups);