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 75cc286485
("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>

configure

@@ -1987,6 +1987,14 @@ if test "$skip_meson" = no; then
   if test "$?" -ne 0 ; then
       error_exit "meson setup failed"
   fi
+else
+  if test -f meson-private/cmd_line.txt; then
+    # Adjust old command line options that were removed
+    # sed -i is not portable
+    perl -i -ne '
+      /^sphinx_build/ && next;
+      print;' meson-private/cmd_line.txt
+  fi
 fi
 
 # Save the configure command line for later reuse.