[web] Added section about using default gcc versions on Ubuntu with Conan.

web/content/docs/devguide/troubleshooting/conan.pandoc

@@ -42,3 +42,18 @@ In `~/.conan/settings.yml` it is defined which compiler versions are supported b
 - when upgrading Conan it creates a file `~/.conan/settings.new.yml` or similar which you can just rename to `~/.conan/settings.yml`
 
 See also: http://docs.conan.io/en/latest/faq/troubleshooting.html#error-invalid-setting
+
+## Something went wrong during building
+
+On Ubuntu always use the system default gcc version! Otherwise if you use a newer compiler
+than the default you may end up using incompatible Conan packages. E.g. when you use gcc 6
+on Ubuntu 16.04 (where gcc 5 is the default) Conan uses packages which are built on Ubuntu
+17.10 (where gcc 6 is the default). Ubuntu 17.10 uses a newer glibc library (2.25) which
+is incompatible to the glibc library on Ubuntu 16.04 (2.23).
+
+In this case you would get linker errors with the Conan VTK library:
+
+```
+error: undefined reference to `getrandom'
+.../expat/vtkexpat/lib/xmlparse.c 694
+```