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>

scripts/meson-buildoptions.sh

@@ -40,6 +40,7 @@ meson_options_help() {
   printf "%s\n" '                           jemalloc/system/tcmalloc)'
   printf "%s\n" '  --enable-module-upgrades try to load modules from alternate paths for'
   printf "%s\n" '                           upgrades'
+  printf "%s\n" '  --enable-plugins         TCG plugins via shared library loading'
   printf "%s\n" '  --enable-rng-none        dummy RNG, avoid using /dev/(u)random and'
   printf "%s\n" '                           getrandom()'
   printf "%s\n" '  --enable-safe-stack      SafeStack Stack Smash Protection (requires'
@@ -401,6 +402,8 @@ _meson_option_parse() {
     --enable-pipewire) printf "%s" -Dpipewire=enabled ;;
     --disable-pipewire) printf "%s" -Dpipewire=disabled ;;
     --with-pkgversion=*) quote_sh "-Dpkgversion=$2" ;;
+    --enable-plugins) printf "%s" -Dplugins=true ;;
+    --disable-plugins) printf "%s" -Dplugins=false ;;
     --enable-png) printf "%s" -Dpng=enabled ;;
     --disable-png) printf "%s" -Dpng=disabled ;;
     --enable-pvrdma) printf "%s" -Dpvrdma=enabled ;;