debian: set libexecdir for deb build

Problem: dh_auto_configure(1) does not seem to set libexecdir
automatically for package builds.

Add -DCMAKE_INSTALL_LIBEXECDIR to get the correct libexecdir for
debian systems in the cmake based build.

debian/rules

@@ -18,6 +18,10 @@
 override_dh_autoreconf:
 	@echo not running autogen.sh on dist product
 
+override_dh_auto_configure:
+	dh_auto_configure -- \
+	  -DCMAKE_INSTALL_LIBEXECDIR=/usr/lib/$(DEB_HOST_MULTIARCH)
+
 override_dh_auto_install:
 	dh_auto_install
 	find . -name '*.la' -delete