configure, meson: synchronize defaults for configure and Meson Rust options

If the defaults for --enable-rust ($rust in configure) and Meson's rust
option are out of sync, incremental builds will pick Meson's default.

This happens because, on an incremental build, configure does not run
Meson, Make does instead.  Meson then gets the command line options
from either coredata.dat (which has everything cached in Python's pickle
format) or cmd_line.txt (slow path when Meson version is upgraded), but
neither knows about the rust option, and the meson_options.txt default
is used.

This will cause have_rust to be true if rustc is available; and the build
to fail because configure did not put a RUST_TARGET_TRIPLE in config-host.mak.

When in the Rust pull request I changed the $rust default from auto
to disabled, I should have made the same change to meson_options.txt;
do it now.

Cc: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241014110113.173946-1-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -1975,7 +1975,7 @@ if test "$skip_meson" = no; then
   test "$pie" = no && meson_option_add -Db_pie=false
 
   # QEMU options
-  test "$rust" != "auto" && meson_option_add "-Drust=$rust"
+  test "$rust" != "disabled" && meson_option_add "-Drust=$rust"
   test "$cfi" != false && meson_option_add "-Dcfi=$cfi" "-Db_lto=$cfi"
   test "$docs" != auto && meson_option_add "-Ddocs=$docs"
   test -n "${LIB_FUZZING_ENGINE+xxx}" && meson_option_add "-Dfuzzing_engine=$LIB_FUZZING_ENGINE"

meson_options.txt

@@ -374,5 +374,5 @@ option('hexagon_idef_parser', type : 'boolean', value : true,
 option('x86_version', type : 'combo', choices : ['0', '1', '2', '3', '4'], value: '1',
        description: 'tweak required x86_64 architecture version beyond compiler default')
 
-option('rust', type: 'feature', value: 'auto',
+option('rust', type: 'feature', value: 'disabled',
        description: 'Rust support')