Use AC_SYS_LARGEFILE

... and include config.h in makekeys.c to get the definition of
_FILE_OFFSET_BITS. Without it, libX11 can fail to build on a file
system with 64-bit inode numbers.

Bug: https://bugs.gentoo.org/550502
Bug: https://bugs.gentoo.org/616140


Signed-off-by: Matt Turner <mattst88@gmail.com>