ignore presence/absence of xsltproc, just use the bundled manuals
which are included in tarball downloads

Index: man/CMakeLists.txt
--- man/CMakeLists.txt.orig
+++ man/CMakeLists.txt
@@ -24,7 +24,9 @@ if(XSLTPROC)
 	compile_manpage("mosquitto-tls.7")
 	compile_manpage("mqtt.7")
 	compile_manpage("mosquitto.8")
+endif()
 
+if(TRUE)
 	install(FILES
 		mosquitto_ctrl.1
 		mosquitto_ctrl_dynsec.1