nftw: Fix build on older distros.

Apparently defining _XOPEN_SOURCE without a value breaks struct timespec
definition from time.h on older distros and the compilation fails with
undeclared struct timespec in safe_file_ops.h.

Fix this by changing it to -D_XOPEN_SOURCE=500 (as suggested in man page
for struct FTW) and do a little cleanup (drop -DLINUX and its ifdefs).

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>