Utilities/Sphinx: Build documentation independently by default

Previously we made the `documentation` target depend on the executable
targets.  There is no real reason to do this other than serializing the
documentation build output in terminals.  Move this dependency behind an
undocumented option so that it is not on by default.

Utilities/Sphinx/CMakeLists.txt

@@ -128,18 +128,20 @@ endforeach()
 
 add_custom_target(documentation ALL DEPENDS ${doc_format_outputs})
 
-foreach(t
-    cmake
-    ccmake
-    cmake-gui
-    cpack
-    ctest
-    )
-  if(TARGET ${t})
-    # Build documentation after main executables.
-    add_dependencies(documentation ${t})
-  endif()
-endforeach()
+if(CMake_SPHINX_DEPEND_ON_EXECUTABLES)
+  foreach(t
+      cmake
+      ccmake
+      cmake-gui
+      cpack
+      ctest
+      )
+    if(TARGET ${t})
+      # Build documentation after main executables.
+      add_dependencies(documentation ${t})
+    endif()
+  endforeach()
+endif()
 
 if(SPHINX_MAN)
   file(GLOB man_rst RELATIVE ${CMake_SOURCE_DIR}/Help/manual