Force use of std::experimental::filesystem with GCC 8

cmake/TestStdFilesystem.cpp

@@ -5,6 +5,10 @@
 #include <experimental/filesystem>
 #endif
 
+#if defined(WALBERLA_USE_STD_FILESYSTEM) && defined(__GLIBCXX__) && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE < 9)
+#error "std:filesystem broken due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90050"
+#endif
+
 int main() {
 #if defined(WALBERLA_USE_STD_FILESYSTEM)
    std::filesystem::path p("/tmp/test.txt");