[CMake][Release] Use @rpath on OS X if possible

CMake >= 2.8.12 can optionally use @rpath to embed dynamic linker paths
into shared libraries on OS X. This greatly improves reliability with
libraries in non-standard locations and should definitely be turned on
by default.

cmake/modules/DuneMacros.cmake

@@ -76,6 +76,9 @@
 # library BASENAME
 #
 
+# Make CMake use rpath on OS X
+cmake_policy(SET CMP0042 NEW)
+
 enable_language(C) # Enable C to skip CXX bindings for some tests.
 
 include(FeatureSummary)