Adding support for libseccomp in configure and Makefile (v8)

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)