Turns out (1) works:

  #include <stdio.h>
  #include <stdlib.h>

  static void begin() __attribute__((constructor));

  void begin() {
    unsetenv("LD_PRELOAD");
  }

Build with:

  gcc -shared -fpie -o library.so library.c

Test:

  LD_PRELOAD=~/library.so env | grep LD_PRELOAD