[cmake] Revert "make use of "enable_examples" in CMakeLists.txt"

This reverts parts of commit 367fd39.
The use of "enable_examples" is in EXAMPLES/CMakeLists.txt. The
advanatge of the solution is that even with disabled
enable_examples the exmaples can be manually built.

CMakeLists.txt

@@ -255,15 +255,13 @@ if(enable_tests)
   add_subdirectory(TESTING)
 endif()
 
-if (enable_examples)
-  add_custom_target(examples)
-  add_subdirectory(EXAMPLE)
-endif()
-
+add_custom_target(examples)
 if (enable_fortran)
   add_subdirectory(FORTRAN)
 endif()
 
+add_subdirectory(EXAMPLE)
+
 if (enable_doc)
    add_subdirectory(DOC)
 endif()