Added a note in compile instructions.

doc/compile.md

@@ -16,6 +16,11 @@ ccmake ..
 make
 ```
 
+If Eigen gives you compilation errors, install the last version from their site and configure the project with
+```
+ccmake .. -DEigen3_DIR=/your/eigen/install/path/eigen/cmake/
+```
+
 If you don't compile tests, you will get an executable called `maxwell_solver`. It takes a single parameter, the path of the Lua configuration script (see in `share` for some examples).
 
 To visualize the results you need VisIt (https://wci.llnl.gov/simulation/computer-codes/visit).
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