Don't use g_memmove().

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Use AC_HEADER_MAJOR.

Instead of checking for sys/mkdev.h headerfile, there is the
AC_HEADER_MAJOR helper for how to get major(), minor(), makedev().

Sinc with GLib efb1701bf3baf6f5b05fd1a7a5a4ff990a7dc460.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>