WIP: fix export/override prefix and ?= first-wins in make variable parser

Two defects in the lint-once resolver's make variable handling:

- The assignment pattern anchored at the start of the name, so
  `export FMT := script/fmt-check` was never collected and `@$(FMT)`
  went unresolved. An optional `export `/`override ` prefix is now
  allowed.
- `?=` assigns only when the name is unset, so the first assignment
  wins. The parser called .set() unconditionally, letting a later
  `FMT ?= script/build` overwrite an earlier `FMT := script/fmt-check`
  and resolve to a command make never runs.

Tests pinning both to follow.

test/packaging/lint-once.test.ts

@@ -159,7 +159,7 @@ const MAKE_CONDITIONAL = /^\s*(?:ifeq|ifneq|ifdef|ifndef|else|endif)\b/;
 // not happen. An unresolved reference is left standing verbatim instead, which
 // is the same dead end as any other unfollowed edge rather than a wrong answer.
 const MAKE_ASSIGNMENT = new RegExp(
-    `^([A-Za-z_][A-Za-z0-9_]*)\\s*(?::=|\\?=|=)\\s*(.*)$`,
+    `^(?:(?:export|override)\\s+)*([A-Za-z_][A-Za-z0-9_]*)\\s*(:=|\\?=|=)\\s*(.*)$`,
 );
 const MAKE_VARIABLE_REFERENCE = /\$[({]([A-Za-z_][A-Za-z0-9_]*)[)}]/g;
 
@@ -181,8 +181,13 @@ const makeVariables = (text: string): Map<string, string> => {
         if (inDefine || raw.startsWith("\t")) continue;
         const assignment = MAKE_ASSIGNMENT.exec(raw.replace(/#.*$/, "").trim());
         if (assignment === null) continue;
-        const [, name, value] = assignment;
-        if (name === undefined || value === undefined) continue;
+        const [, name, operator, value] = assignment;
+        if (name === undefined || operator === undefined) continue;
+        if (value === undefined) continue;
+        // `?=` assigns only when the name has no value yet, so the first one
+        // wins where `:=` and `=` let the last one win. Overwriting here would
+        // resolve the reference to a string make never uses.
+        if (operator === "?=" && variables.has(name)) continue;
         // A value that is itself a reference is the recursive case, and is not
         // resolved. Recording it would hand the substitution below a string it
         // cannot finish expanding.