configure, meson: move --enable-plugins to meson

While the option still needs to be parsed in the configure script
(it's needed by tests/tcg, and also to decide about recursing
into contrib/plugins), passing it to Meson can be done with -D
instead of using config-host.mak.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1712,10 +1712,6 @@ case $targetos in
     ;;
 esac
 
-if test "$plugins" = "yes" ; then
-    echo "CONFIG_PLUGIN=y" >> $config_host_mak
-fi
-
 if test -n "$gdb_bin"; then
     gdb_version=$($gdb_bin --version | head -n 1)
     if version_ge ${gdb_version##* } 9.1; then
@@ -1909,6 +1905,7 @@ if test "$skip_meson" = no; then
   test "$cfi" != false && meson_option_add "-Dcfi=$cfi"
   test "$docs" != auto && meson_option_add "-Ddocs=$docs"
   test -n "${LIB_FUZZING_ENGINE+xxx}" && meson_option_add "-Dfuzzing_engine=$LIB_FUZZING_ENGINE"
+  test "$plugins" = yes && meson_option_add "-Dplugins=true"
   test "$qemu_suffix" != qemu && meson_option_add "-Dqemu_suffix=$qemu_suffix"
   test "$smbd" != '' && meson_option_add "-Dsmbd=$smbd"
   test "$tcg" != enabled && meson_option_add "-Dtcg=$tcg"