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>
8 files changed