Commit 5c9b4f3163 inadvertently broke
cross builds by making Meson process the doc/misc/meson.build file even
when sphinx-build is not found in PATH. The doc/misc/meson.build file
defines targets that require a non-native executable, cfg_test, in order
to be built.
Fix by reverting to only processing the doc/misc/ subdirectory when
sphinx-build is found in PATH and moving the relevant alias_target()
method call so that the build targets depending on a non-native
executable are only defined if sphinx-build is found in PATH.
ee5897743d