[cmake] install files added by merge request !16

Since !16 (merged) was merged by mistake, using an installed version of dune-common does not work anymore, even without actually using VC, as the file cmake/modules/AddVcFlags.cmake does not get installed.

So if we keep VC, I suggest to install the new files added in merge request !16 (merged).

cmake/modules/CMakeLists.txt

@@ -5,6 +5,7 @@ install(FILES
   AddPTScotchFlags.cmake
   AddSuiteSparseFlags.cmake
   AddUMFPackFlags.cmake
+  AddVcFlags.cmake
   CheckCXXFeatures.cmake
   CheckForPthreads.c
   CMakeBuiltinFunctionsDocumentation.cmake