cmake 3.12 (fixup for boost constraints)

+ cmake >= 3.12 is required for finding correctly Boost 1.67
+ cmake 3.16 is present in Debian stretch-backports, but it requires libarchive 3.3.3 (not present in stretch / stretch-backports) - which can be compiled from Buster src to Debian package

debian/control

@@ -7,7 +7,7 @@ Section: misc
 Standards-Version: 3.6.1
 X-Python-Version: >= 2.6
 Build-Depends: bison,
-	ccache, cmake (>=3.2),
+	ccache, cmake (>=3.12),
 	debhelper (>= 5.0.38), dh-python, docbook, docbook-dsssl,
 	flex,
 	gettext,