Fix regression: redefine popen only for GlibC.
Fix regression: redefine popen only for GlibC.

solderzzc/fakechroot,giuliolunati/fakechroot,solderzzc/fakechroot,solderzzc/fakechroot,andrewgregory/fakechroot,solderzzc/fakechroot,solderzzc/fakechroot,andrewgregory/fakechroot,giuliolunati/fakechroot,andrewgregory/fakechroot,giuliolunati/fakechroot