stdenv: make _accumFlagsArray independent of structuredAttrs

structuredAttrs was used here to make an assumption about the type of
the named variables passed as arguments.  This can be done better by
looking at the actual types of those variables.

This gives a bit more backwards compatibility as well: Once you turn to
structuredAttrs, you should still be able to pass a bare string instead
of a list and have it behave as a whitespace-separated string like
before.

pkgs/stdenv/generic/setup.sh

@@ -336,34 +336,24 @@ appendToVar() {
 
 # Accumulate into `flagsArray` the flags from the named variables.
 #
-# If __structuredAttrs, the variables are all treated as arrays
-# and simply concatenated onto `flagsArray`.
-#
-# If not __structuredAttrs, then:
-#   * Each variable is treated as a string, and split on whitespace;
-#   * except variables whose names end in "Array", which are treated
-#     as arrays.
+# Arrays are simply concatenated, strings are split on whitespace.
 _accumFlagsArray() {
-    local name
-    if [ -n "$__structuredAttrs" ]; then
-        for name in "$@"; do
-            local -n nameref="$name"
-            flagsArray+=( ${nameref+"${nameref[@]}"} )
-        done
-    else
-        for name in "$@"; do
+    local name type
+    for name in "$@"; do
+        if type=$(declare -p "$name" 2> /dev/null); then
             local -n nameref="$name"
-            case "$name" in
-                *Array)
-                    # shellcheck disable=SC2206
-                    flagsArray+=( ${nameref+"${nameref[@]}"} ) ;;
+            case "${type#* }" in
+                -A*)
+                    echo "_accumFlagsArray(): ERROR: trying to use _accumFlagsArray on an associative array." >&2
+                    return 1 ;;
+                -a*)
+                    flagsArray+=( "${nameref[@]}" ) ;;
                 *)
                     # shellcheck disable=SC2206
                     flagsArray+=( ${nameref-} ) ;;
             esac
-        done
-    fi
-
+        fi
+    done
 }
 
 # Add $1/lib* into rpaths.