Only add the doc target if doxygen and sphinx-build are available

doc/CMakeLists.txt

+foreach (program doxygen sphinx-build)
+  find_program(${program} ${program})
+  if (NOT ${program})
+    message(STATUS "Target 'doc' disabled (requires ${program})")
+    return ()
+  endif ()
+endforeach ()
+
 add_custom_target(doc
-  COMMAND doxygen
-  COMMAND rm -rf html
-  COMMAND sphinx-build -b html . html)
+  COMMAND ${doxygen}
+  COMMAND ${sphinx-build} -b html . html)