Fix icecc-create-env for Darwin

* Parse outputs from otool that contains @executable_path
     in the libs path like: @executable_path/libllvmgcc.dylib
   * Check /bin/true before add it.
   * Fix parse to command `as -micha`
     $ /usr/bin/as -micha
     "FATAL:/usr/bin/../libexec/as/x86_64/as: I don't understand ..."
   * Check if exists /etc/ld.so.conf before parse its content.

Signed-off-by: Ragner Magalhaes <ragner.magalhaes@gmail.com>