Add travis build with NAMED_SUB_FORWARD_LIST

We add this step to check that CBMC compiles fine with this flag, and
run relevant unit tests for irept.

.travis.yml

@@ -174,6 +174,33 @@ jobs:
         - CCACHE_CPP2=yes
       script: echo "Not running any tests for a debug build."
 
+    # cmake build using g++-7, enable NAMED_SUB_IS_FORWARD_LIST
+    - stage: Test different OS/CXX/Flags
+      os: linux
+      sudo: false
+      compiler: gcc
+      cache: ccache
+      env:
+        - BUILD_SYSTEM=cmake
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+            - jq
+      before_install:
+        - mkdir bin
+        - ln -s /usr/bin/gcc-7 bin/gcc
+        - ln -s /usr/bin/g++-7 bin/g++
+      install:
+        - ccache -z
+        - ccache --max-size=1G
+        - cmake -H. -Bbuild '-DCMAKE_BUILD_TYPE=Release' '-DCMAKE_CXX_COMPILER=/usr/bin/g++-7' '-DCMAKE_CXX_FLAGS=-DNAMED_SUB_IS_FORWARD_LIST'
+        - git submodule update --init --recursive
+        - cmake --build build -- -j4
+      script: (cd build; bin/unit "[core][irept]")
+
     # cmake build using g++-7, enable CMAKE_USE_CUDD
     - stage: Test different OS/CXX/Flags
       os: linux