Avoid compilation failure on s390

Avoid compilation failure on s390

To export LinuxSpecific stuff for labels, in this case fdasd, we now
provide arch/linux.h. This aids in avoiding code duplication.

* configure.ac: add BUILD_LINUX conditional.
* libparted/Makefile.am: conditionally use linux.h.
* libparted/arch/linux.c: move LinuxSpecific stuff to linux.h.
* libparted/arch/linux.h: New file.
* libparted/labels/Makefile.am: include from libparted as well.
* libparted/labels/dasd.c: use arch/linux.h and use dev->path for
  error message.
* libparted/labels/fdasd.c: include linux.h and drop an unused
  variable.