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().

https://bugzilla.gnome.org/show_bug.cgi?id=712314
AIX splice() is something different

AIX does provide the splice() symbol, but this does have different
signature and usecase than what Linux may provide.

https://bugzilla.gnome.org/show_bug.cgi?id=712314