[cmake] work around a race in Sphinx.

[brooks]

Summary

Create the target directory before calling sphinx to put files in it.
Sphinx has a race where it fails with an unhandled exception if it
decides to create a directory and then someone else gets there first.

[brooks]

I've received multiple reports of this issue from FreeBSD ports users, but have never encountered it myself. https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227460

[brooks]

• Correct whitespace.

[mgorny]

For the record, this has been fixed upstream: https://github.com/sphinx-doc/sphinx/pull/4282. However, I don't know which release version (if any) has the fix.

[brooks]

Should no longer be needed

cmake/modules/AddSphinxTarget.cmake

function (add_sphinx_target builder project)
 
   if (SPHINX_WARNINGS_AS_ERRORS)
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "-W")
   else()
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "")
   endif()
 
   add_custom_target(${SPHINX_TARGET_NAME}
+                    COMMAND ${CMAKE_COMMAND}
+                            -E make_directory ${SPHINX_BUILD_DIR}
                     COMMAND ${SPHINX_EXECUTABLE}
                             -b ${builder}
                             -d "${SPHINX_DOC_TREE_DIR}"
                             -q # Quiet: no output other than errors and warnings.
                             ${SPHINX_WARNINGS_AS_ERRORS_FLAG} # Treat warnings as errors if requested
                             "${CMAKE_CURRENT_SOURCE_DIR}" # Source
                             "${SPHINX_BUILD_DIR}" # Output
                     COMMENT