setup the unsupported directory structure.

The unsupported module documentation is automatically generated in:
  build/doc/unsupported/
with bidirectional cross references.
I leave a class Foo in AdolcForward module to illustrate the
cross-reference behavior. I will remove it in the next commit.

unsupported/Eigen/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(Eigen_HEADERS AdolcForward)
+
+install(FILES
+  ${Eigen_HEADERS}
+  DESTINATION ${INCLUDE_INSTALL_DIR}/unsupported/Eigen
+  )
+
+add_subdirectory(src)