bug #1237: Redefine eigen_assert instead of disabling assertions for documentation snippets

doc/snippets/CMakeLists.txt

diff --git a/doc/snippets/CMakeLists.txt b/doc/snippets/CMakeLists.txt
index 8cc6df0..1baf32f 100644
--- a/doc/snippets/CMakeLists.txt
+++ b/doc/snippets/CMakeLists.txt
@@ -24,6 +24,3 @@
   set_source_files_properties(${CMAKE_CURRENT_BINARY_DIR}/${compile_snippet_src}
                               PROPERTIES OBJECT_DEPENDS ${snippet_src})
 endforeach(snippet_src)
-
-ei_add_target_property(compile_tut_arithmetic_transpose_aliasing COMPILE_FLAGS -DEIGEN_NO_DEBUG)
-ei_add_target_property(compile_TopicAliasing_mult4 COMPILE_FLAGS -DEIGEN_NO_DEBUG) # workaroud for Bug 1237

doc/snippets/compile_snippet.cpp.in

diff --git a/doc/snippets/compile_snippet.cpp.in b/doc/snippets/compile_snippet.cpp.in
index fdae39b..d63f371 100644
--- a/doc/snippets/compile_snippet.cpp.in
+++ b/doc/snippets/compile_snippet.cpp.in
@@ -1,5 +1,8 @@
-#include <Eigen/Eigen>
+static bool eigen_did_assert = false;
+#define eigen_assert(X) if(!eigen_did_assert && !(X)){ std::cout << "### Assertion raised in " << __FILE__ << ":" << __LINE__ << ":\n" #X << "\n### The following would happen without assertions:\n"; eigen_did_assert = true;}
+
 #include <iostream>
+#include <Eigen/Eigen>
 
 #ifndef M_PI
 #define M_PI 3.1415926535897932384626433832795