Simplify CMake.HTML documentation test command line

Drop the "cmake -E chdir" wrapper and instead pass the DTD directory to
xmllint's --path option using url encoding.  While at it, move the
XHTML1 DTD to "Utilities/xml/xhtml1" to make room for additional DTDs.

Utilities/CMakeLists.txt

@@ -141,15 +141,15 @@ if(BUILD_TESTING)
     execute_process(COMMAND ${LIBXML2_XMLLINT_EXECUTABLE} --help
       OUTPUT_VARIABLE _help ERROR_VARIABLE _err)
     if("${_help}" MATCHES "--path" AND "${_help}" MATCHES "--nonet")
-      # We provide the XHTML DTD and its dependencies in the 'xml'
+      # We provide DTDs in the 'xml' directory so that xmllint can run without
-      # directory so that xmllint can run without network access.
+      # network access.  Note that xmllints's --path option accepts a
-      # However, it's --path option accepts a space-separated list of
+      # space-separated list of url-encoded paths.
-      # paths so it cannot handle spaces in the path to the source tree.
+      set(_dtd_dir "${CMAKE_CURRENT_SOURCE_DIR}/xml")
-      # Therefore we run the tool with the current work directory set to
+      string(REPLACE " " "%20" _dtd_dir "${_dtd_dir}")
-      # the 'xml' directory and use '.' as the path.
+      string(REPLACE ":" "%3A" _dtd_dir "${_dtd_dir}")
       add_test(CMake.HTML
-        ${CMAKE_CMAKE_COMMAND} -E chdir ${CMAKE_CURRENT_SOURCE_DIR}/xml
+        ${LIBXML2_XMLLINT_EXECUTABLE} --valid --noout --nonet
-        ${LIBXML2_XMLLINT_EXECUTABLE} --valid --noout --nonet --path .
+        --path ${_dtd_dir}/xhtml1
         ${HTML_FILES}
         )
     endif()