title: Fortran precision default with/out compiler flag
body: <p>I'm having trouble with the precision of constant numerics in Fortran.</p>
  
  <p>Do I need to write every <code>0.1</code> as <code>0.1d0</code> to have double precision? I know the compiler has a flag such as <code>-fdefault-real-8</code> in gfortran that solves this kind of problem.  Would it be a portable and reliable way to do? And how could I check if the flag option actually works for my code?</p>
  
  <p>I was using F2py to call Fortran code in my Python code, and it doesn't report an error even if I give an unspecified flag, and that's what's worrying me.</p>