build: Fix pnglibconf for the awk implementations that lack asort

scripts/pnglibconf/checksym.awk

@@ -94,8 +94,8 @@ END {
    }
 
    # Sort and print the symbols.
-   asort(sorted_symbols)
-   for (i = 1; i <= length(sorted_symbols); i++) {
+   num = do_sort(sorted_symbols)
+   for (i = 1; i <= num; i++) {
       print " " sorted_symbols[i] >of
    }
 
@@ -106,3 +106,24 @@ END {
 
    # TODO: Check for symbols that are both defined and removed.
 }
+
+# Portable wrapper around the gawk-specific asort function.
+function do_sort(arr) {
+   if ("identifiers" in PROCINFO && "asort" in PROCINFO["identifiers"]) {
+      # Use asort (available in gawk).
+      return asort(arr)
+   }
+   # Fall back to a hand-made insertion sort, which should be fast enough
+   # for our use case.
+   num = length(arr)
+   for (i = 2; i <= num; i++) {
+      key = arr[i]
+      j = i - 1
+      while (j > 0 && arr[j] > key) {
+         arr[j + 1] = arr[j]
+         j = j - 1
+      }
+      arr[j + 1] = key
+   }
+   return num
+}