Test the with-rust and without-rust distcheck variants

.travis.yml

@@ -100,7 +100,9 @@ matrix:
     - compiler: gcc
       env: COVERAGE_OPTIONS="--enable-coverage"
     - compiler: gcc
-      env: DISTCHECK="yes"
+      env: DISTCHECK="yes" RUST_OPTIONS=""
+    - compiler: gcc
+      env: DISTCHECK="yes" RUST_OPTIONS="--enable-rust --enable-cargo-online-mode"
     ## The "sudo: required" forces non-containerized builds, working
     ## around a Travis CI environment issue: clang LeakAnalyzer fails
     ## because it requires ptrace and the containerized environment no