Propagate LD_FLAGS from Cmake into sac2crc.

We set up LD_FLAGS in cmake/sac2c/config.cmake that should capture differences in OS linking. However, these never get used neither when building sac2c nor in sac2crc. This, for example, causes a failure on ubuntu when compiling the following sac program:

    use Array:all;
    use StdIO:all;

    int main () {
      a = [1,2,3,4];
      b = [a,a];
      a = modarray( modarray( a, [0], 0), [1], 0);
      print(a);
      return 0;
    }

Using the following sac2c flags:

    sac2c -v0 -O0 -noprelude -noinl -specmode aud a.sac

Adding -Xl '-Wl,--allow-shlib-undefined' solves the problem, however this very flag is already defined in LD_FLAGS.