doc: install csexec(1) man page only if csexec is installed

Author: kdudka

doc/CMakeLists.txt

@@ -30,7 +30,9 @@ endmacro()
 # build and install man pages (if asciidoc is available)
 find_program(A2X a2x)
 if(A2X)
-    add_man_page(csexec)
+    if(TARGET csexec)
+        add_man_page(csexec)
+    endif()
     add_man_page(cswrap)
     add_custom_target(doc ALL DEPENDS ${man_pages})
 endif()