autotools: Split Docbook condition from AM_XSLTPROCFLAGS

Doing that allows us to use different conditions in local-rules.mk to
build website’s manpages.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
.gitignore: Ignore local-rules.mk

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
autotools: Support merged libsystemd (>= 209)

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>