The tests in xattr.c assume an FS with xattr support

[Gabor Buella]

The tests in tests/preload/xattr/xattr.c expect setxattr to work on paths not mounted under pmemfile.
This caused a problem when I had TEST_DIR=/dev/shm:

$ cf -R preload_xa
Test project /home/tej/code/pmemfile/build
    Start 68: preload_xattr
1/1 Test #68: preload_xattr ....................***Failed    0.09 sec
preload_xattr: setxattr failed: Operation not supported
CMake Error at /home/tej/code/pmemfile/tests/helpers.cmake:105 (message):
  /home/tej/code/pmemfile/build/tests/preload/preload_xattr /dev/shm//xattr
  failed: 3
Call Stack (most recent call first):
  /home/tej/code/pmemfile/tests/preload/xattr/xattr.cmake:45 (execute)

This test failure looks exactly as if libpmemfile would return ENOTSUP, but in this case the kernel returned this error code.

[Marcin Ślusarz]

Let's detect whether underlying fs supports xattrs at cmake time and add option to disable them.