build-sys: add HAVE_IPPROTO_MPTCP

The QAPI schema shouldn't rely on C system headers #define, but on
configure-time project #define, so we can express the build condition in
a C-independent way.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210907121943.3498701-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index fbdab30..60f4f45 100644
--- a/meson.build
+++ b/meson.build
@@ -1374,6 +1374,8 @@
                      cc.has_header_symbol('getopt.h', 'optreset'))
 config_host_data.set('HAVE_UTMPX',
                      cc.has_header_symbol('utmpx.h', 'struct utmpx'))
+config_host_data.set('HAVE_IPPROTO_MPTCP',
+                     cc.has_header_symbol('netinet/in.h', 'IPPROTO_MPTCP'))
 
 # has_member
 config_host_data.set('HAVE_SIGEV_NOTIFY_THREAD_ID',