libgfapi: Fix to compilation failure/warnings for examples/glfsxmp.c

The comilation of glfsxmp.c i.e. "make glfsxmp" was failing
with "undefined reference to `clock_gettime'" error.

Fixed all compilation warnings for unused variables and
wrong formart specifier in printf e.g. : "format ‘%d’ expects
argument of type ‘int’, but argument 3 has type ‘__syscall_slong_t’
[-Wformat]"

Also added the compilation steps for glfsxmp.c in the
README file

Change-Id: I399ec7c9ad7b6412463c89099d63922caef6749e
Signed-off-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-on: http://review.gluster.org/6085
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>