Only check for asciidoc warnings in "make check" if
ASCIIDOC_WARNINGS_CHECK is set. Enable it in jenkins.sh by using the
new ./configure --enable-asciidoc-warnings-check parameter (similar to
--enable-werror in other Osmocom projects' configure.ac files).
Related: OS#4140
Change-Id: Iac993a0d1b17205397a1f1ef1a7bd3f9df739e36