build: avoid obsolete AC_HELP_STRING

build: avoid obsolete AC_HELP_STRING

Autoconf states that AC_HELP_STRING is obsolete, and that new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: Eric Blake <eblake@redhat.com>