build-sys: fix conftest.c to work on arm

All tests for compiler support were failing with:

/usr/bin/ld: /tmp/ccwnVc2A.o: relocation R_ARM_MOVW_ABS_NC against `a' can not be used when making a shared object; recompile with -fPIC
util.h: include missing.h, for struct file_handle

This breaks udev-builtin-btrfs.c, which reinvents some of missing.h,
so use missing.h there too.

[zj: removed #include "config.h" and wrapped #include <linux/btrfs.h>
     in ifdef HAVE_LINUX_BTRFS_H as discussed on the mailing list.]
Move BTRFS_IOC_DEVICES_READY to missing.h