Add HAVE_LIBCAP

Add HAVE_LIBCAP

Currently it is impossible to build lxc with --disable-capabilities if
the user has libcap-dev installed on his system as:
 - calls to cap_xxx functions are not protected by HAVE_LIBCAP defines.
 The whole file is only protected by HAVE_SYS_CAPABILITY_H.
 - AC_CHECK_LIB default action-if-found is overriden by [true] so
 HAVE_LIBCAP is never written to config.h

This patch replaces all HAVE_SYS_CAPABILITY_H checks by HAVE_LIBCAP
checks (fix #1361)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>