BSD: Run post-install script to use efficient engine.

FreeBSD needs sgid kmem in order to access KVM. If this isn't
set, we use a slow-assed fallback method.

src/bin/meson.build

@@ -17,3 +17,7 @@ executable('evisum', [
    dependencies        : [ deps, deps_os ],
    gui_app             : true,
    install             : true)
+
+if host_os == 'freebsd' or host_os == 'dragonfly'
+   meson.add_install_script('perms.sh')
+endif

src/bin/perms.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# We do this to allow us to poll for processes efficiently.
+# Specifically for FreeBSD and DragonFlyBSD.
+chown root:kmem "${DESTDIR}/${MESON_INSTALL_PREFIX}/bin/evisum"
+chmod g+s "${DESTDIR}/${MESON_INSTALL_PREFIX}/bin/evisum"