manual: llvmlite-artiq conda package needs a recent libstdc++6

doc/manual/installing.rst

@@ -42,6 +42,25 @@ Then you can install the ARTIQ package, it will pull all the necessary dependenc
 
     $ conda install artiq
 
+.. note::
+    You need to have a recent libstdc++6.
+    If you are installing on e.g. Ubuntu 14.04 LTS: you need to update your
+    libstdc++6.
+
+    If the following command outputs `GLIBCXX_3.4.20`, then you don't need to
+    upgrade::
+
+        $ strings /usr/lib/x86_64-linux-gnu/libstdc++.so.6 | grep GLIBCXX_3.4.20
+        GLIBCXX_3.4.20
+
+    Adapt the previous command to the location of libstdc++ on your system.
+
+    To upgrade, on Ubuntu you need to do the following::
+
+        $ sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+        $ sudo apt-get update
+        $ sudo apt-get install libstdc++6
+
 Preparing the core device FPGA board
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^