[cmake] Include project name in Sphinx doctree dir to fix race conditions

[mgorny]

Summary

Modify add_sphinx_target() to include the project name alongside builder
in Sphinx doctree directory. This aims to avoid crashes due to race
conditions between multiple Sphinx instances running in parallel that
attempt to create or read that directory simultaneously.

This problem has originally been addressed in rL283188. However, that
commit presumed that there will be only one target per builder being
run. However, rL314863 introduced a second manpage target, reintroducing
the race condition.
-

[beanz]

LGTM.

cmake/modules/AddSphinxTarget.cmake

 # Handy function for creating the different Sphinx targets.
 #
 # ``builder`` should be one of the supported builders used by
 # the sphinx-build command.
 #
 # ``project`` should be the project name
 function (add_sphinx_target builder project)
   set(SPHINX_BUILD_DIR "${CMAKE_CURRENT_BINARY_DIR}/${builder}")
-  set(SPHINX_DOC_TREE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_doctrees-${builder}")
+  set(SPHINX_DOC_TREE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_doctrees-${project}-${builder}")
   set(SPHINX_TARGET_NAME docs-${project}-${builder})
 
   if (SPHINX_WARNINGS_AS_ERRORS)
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "-W")
   else()
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "")
   endif()