- Cannot call m4_append() with different input based on shell 'if' branch:
    if $test; then
      m4_append(MACRO,[if_true])
    else
      m4_append(MACRO,[if_false])
    fi
  results in MACRO with both [if_true] and [if_false] appended.
- Flags appended to ${ac_link} must still be appended to LDFLAGS, since
  ${ac_link} is not used to create the Makefile linker command.
- Refs #4583
Original: 6940939e6d48fdad8eb2576c97dc96b1670da3e5