build-sys: Enable libsoup3 by default if installed

In f39 we switched to libsoup3 by default; this ensures our CI
picks that up automatically so we still have ostree-trivial-httpd.

configure.ac

@@ -155,7 +155,7 @@ AS_IF([test x$enable_http2 != xno ], [
 SOUP3_DEPENDENCY="libsoup-3.0 >= 3.0.0"
 AC_ARG_WITH(soup3,
 	    AS_HELP_STRING([--with-soup3], [Use libsoup3 @<:@default=no@:>@]),
-	    [], [with_soup3=no])
+	    [], [with_soup3=check])
 AS_IF([test x$with_soup3 != xno], [
     PKG_CHECK_MODULES(OT_DEP_SOUP3, $SOUP3_DEPENDENCY)
     with_soup3=yes