cmake: Preserve environment build FLAGS

To avoid dropping hardening build flags when built on Debian/RHEL.

Change-Id: I6b1c70d6a6e9151d6cd82d1b5411afe1ef799a1b
cmake: Assist troubleshooting asciidoc related FTBFS

Increase verbosity so conversion errors would appear in build log.

"--keep-artefacts" leaves intermedient .xml files behind (may be handy in
case of validation problems).

Change-Id: Iacbb8e852f785bea8a42ec72210bafc1ea2ddaf5