configure script doesn't fail when --enable-generate-docs is used and docbook-style-xsl is not installed

when kea is configured with: ./configure --enable-generate-docs --with-log4cplus=/opt/log4cplus but docbook-style-xsl is not installed, configure script finish with success and process of building documentation fail:

[jenkins@fedora27-64-2 tools]$ cd ../doc/
[jenkins@fedora27-64-2 doc]$ make
Making all in guide
make[1]: Entering directory '/home/jenkins/workspace/kea-pipeline-multiconf-admin/doc/guide'
/usr/bin/elinks -dump -no-numbering -no-references kea-guide.html > kea-guide.txt
../../tools/system_messages -o kea-messages.xml \
`find ../.. -name "*.mes" -print`
/usr/bin/xsltproc --novalid --xinclude --nonet \
	--path ../../doc \
	-o kea-messages.html \
	--stringparam generate.toc "book toc" \
	--stringparam html.stylesheet kea-guide.css \
	http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl \
	kea-messages.xml
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl
make[1]: *** [Makefile:644: kea-messages.html] Error 4
make[1]: Leaving directory '/home/jenkins/workspace/kea-pipeline-multiconf-admin/doc/guide'
make: *** [Makefile:532: all-recursive] Error 1

configure script should detect missing package and exit with error