Adding support for libseccomp in configure and Makefile (v8)

Adding basic options to the configure script to use libseccomp or not.
The default is set to 'no'. If the flag --enable-libseccomp is used, the
script will check for its existence using pkg-config.

Signed-off-by: Eduardo Otubo <otubo@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
v1 -> v2:
 - As I removed all the code related to seccomp from vl.c, I created
   qemu-seccomp.[ch].
 - Also making the configure script to add the specific line to
   Makefile.obj in order to compile with appropriate support to seccomp.

v2 -> v3:
 - Removing the line from Makefile.obj and adding it to Makefile.objs.
 - Marking libseccomp default option to 'yes' in the configure script.

v3 -> v8:
 - fix configure probe if libseccomp isn't available (aliguori)
