removed several warnings, by including Eigen as a system library

cmake/modules/FindEigen3.cmake

@@ -103,6 +103,6 @@ endif(EIGEN3_INCLUDE_DIR)
 if(EIGEN3_FOUND)
   dune_register_package_flags(
     COMPILE_DEFINITIONS "ENABLE_EIGEN3=1" "DEC_HAS_EIGEN=1"
-    INCLUDE_DIRS "${EIGEN3_INCLUDE_DIR}")
+  )
+  include_directories(SYSTEM "${EIGEN3_INCLUDE_DIR}")
 endif()
-

dune/dec/utility/Filesystem.cpp

@@ -18,6 +18,9 @@
 
 #include <dune/dec/common/Output.hpp>
 
+template <class... Args>
+void inline _ignore_(Args&&...) {}
+
 namespace Dec { namespace filesystem {
 
 std::string path::string() const
@@ -120,7 +123,7 @@ bool path::is_directory() const
 path current_path()
 {
   char cwd_[FILENAME_MAX];
-  GET_CURRENT_DIR(cwd_, sizeof(cwd_));
+  _ignore_(GET_CURRENT_DIR(cwd_, sizeof(cwd_)));
   std::string cwd(cwd_);
   return { trim(cwd) };
 }