Commit 292d3f2d fixed the build with libselinux 2.3, but missed
some suggestions by eblake

https://www.redhat.com/archives/libvir-list/2014-May/msg00977.html

This patch changes the macro introduced in 292d3f2d to either be
empty in the case of newer libselinux, or contain 'const' in the
case of older libselinux.  The macro is then used directly in
tests/securityselinuxhelper.c.

Several function signatures changed in libselinux 2.3, now taking
a 'const char *' instead of 'security_context_t'.  The latter is
defined in selinux/selinux.h as

  typedef char *security_context_t;
Signed-off-by: NEric Blake <eblake@redhat.com>

Autoconf states that AC_HELP_STRING is obsolete, and that new
programs should use AS_HELP_STRING.  We also had instances of
not properly quoting the macro usage, and not relying on autoconf's
word-wrapping abilities to avoid long lines.  I validated that this
commit has no impact to the generated configure file.

* configure.ac (AC_ARG_WITH, AC_ARG_ENABLE): Autoconf recommends
the use of AS_HELP_STRING.  Also, use proper quoting and wrap long
lines.
* m4/virt-apparmor.m4 (LIBVIRT_CHECK_APPARMOR): Likewise.
* m4/virt-selinux.m4 (LIBVIRT_CHECK_SELINUX): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>

Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>