tools: Use AC_SYS_LARGEFILE instead of calling getconf(1)

tools: Use AC_SYS_LARGEFILE instead of calling getconf(1)

getconf is not cross-compile friendly since it reports the features of the host
and not the target. There doesn't appear to be a $triplet-getconf.

AC_SYS_LARGEFILE arranges for #defines to appear in config.h however Xen's
build system expects these to be part of C{PP}FLAGS. Since I'm not confident
that everything in Xen includes config.h I instead arrange for the result of
running AC_SYS_LARGERFILE to end up in CFLAGS.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>