fix build errors
Merge pull request #69 from UGerhards/v5-stable-fix3

fix build errors
improve mmexternal doc
Merge branch 'master' of https://github.com/rsyslog/rsyslog-doc
Merge branch 'v5-stable' into v7-stable
Merge branch 'v7-stable' into v8-stable
Merge branch 'v8-stable'