Ensure "CMAKE_INSTALL_PREFIX" is properly passed to inner project.

The value of "ep_install_dir" is reserved for regular
external project. The user should still be able to set
CMAKE_INSTALL_PREFIX when configuring CTK and have it
installed when he expects.

See issue #65

SuperBuild.cmake

@@ -157,7 +157,7 @@ ExternalProject_Add(${proj}
     -DCMAKE_C_FLAGS_INIT:STRING=${CMAKE_C_FLAGS_INIT}
     -DADDITIONAL_C_FLAGS:STRING=${ADDITIONAL_C_FLAGS}
     -DADDITIONAL_CXX_FLAGS:STRING=${ADDITIONAL_CXX_FLAGS}
-    -DCMAKE_INSTALL_PREFIX:PATH=${ep_install_dir}
+    -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}
     -DCTK_BUILD_SHARED_LIBS:BOOL=${CTK_BUILD_SHARED_LIBS}
     ${CMAKE_OSX_EXTERNAL_PROJECT_ARGS}
     -DDOCUMENTATION_ARCHIVES_OUTPUT_DIRECTORY:PATH=${DOCUMENTATION_ARCHIVES_OUTPUT_DIRECTORY}