Environment variables in Postgres C extension

I cannot get environment variable in my PostgreSQL C extension code.

For example, this function always returns 111:

#include "postgres.h"
#include "fmgr.h"
#include <stdlib.h>

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(myinc);
Datum
myinc(PG_FUNCTION_ARGS)
{
  int32 arg = PG_GETARG_INT32(0);
  char* envar = getenv("MYINC");
  if (envar) {
    PG_RETURN_INT32(arg + atoi(envar));
  } else {
    PG_RETURN_INT32(111);
  }
}

While this C program works as expected, it prints whatever MYINC is:

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

int main()
{
  printf("MYINC: %s", getenv("MYINC"));
  return 0;
}