configure: fix backwards-compatibility for meson sphinx_build option

configure: fix backwards-compatibility for meson sphinx_build option

Reintroduce the cmd_line.txt mangling to remove the sphinx_build option
when rerunning meson.  The mechanism was removed in commit 75cc28648574
("configure: remove backwards-compatibility code", 2023-01-11) because
the fixups were obsolete at the time; however, the Meson deprecation
mechanism doesn't quite work when options are finally removed, so we
need to bring it back.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>