make bugged gcc work with DEBUG_ONLY macro

cmake/modules/XtCompilerSupport.cmake

@@ -67,3 +67,8 @@ check_cxx_source_compiles("
       return 0;
     };
 " HAVE_MAP_EMPLACE)
+
+check_cxx_source_compiles("
+    void foo([[maybe_unused]] arg) {}
+    int main(void){};
+" HAS_WORKING_UNUSED_ATTRIBUTE)

dune/xt/common/unused.hh

@@ -15,13 +15,21 @@
 #ifndef NDEBUG
 #  define DXTC_DEBUG_ONLY
 #else
-#  define DXTC_DEBUG_ONLY [[maybe_unused]]
+#  if HAS_WORKING_UNUSED_ATTRIBUTE
+#    define DXTC_DEBUG_ONLY [[maybe_unused]]
+#  else
+#    define DXTC_DEBUG_ONLY __attribute__((unused))
+#  endif
 #endif
 
 #if defined(HAVE_MPI) && HAVE_MPI
 #  define DXTC_MPI_ONLY
 #else
-#  define DXTC_MPI_ONLY [[maybe_unused]]
+#  if HAS_WORKING_UNUSED_ATTRIBUTE
+#    define DXTC_DEBUG_ONLY [[maybe_unused]]
+#  else
+#    define DXTC_MPI_ONLY __attribute__((unused))
+#  endif
 #endif
 
 #endif // DXTC_UNUSED_HH