Add dependency from CMakeLists.txt since it is used to get version.

Author: vitaut

doc/CMakeLists.txt

@@ -2,5 +2,6 @@ add_custom_command(OUTPUT html/index.html
   COMMAND doxygen
   COMMAND rm -rf html
   COMMAND sphinx-build -b html . html
-  DEPENDS ../format.h Doxyfile conf.py index.rst _templates/layout.html)
+  DEPENDS ../CMakeLists.txt ../format.h Doxyfile
+          conf.py index.rst _templates/layout.html)
 add_custom_target(doc DEPENDS html/index.html)