CGAL_ENABLE_TESTING has three definitions

`CGAL_ENABLE_TESTING` is defined three times:
- once in `/CMakeLists.txt` that is the root of the CMake tree for a Git layout
- once in `Installation/CMakeLists.txt` that is the root of the CMake
  for a release tarball,
- and once in `CGAL_add_test.cmake`, for consumers of `CGALConfig.cmake`.

Two changes:
- Add the default value `${BUILD_TESTING}` to all three definitions
- Modify the comment

CMakeLists.txt

@@ -46,9 +46,10 @@ option(
 
 message("== CMake setup (DONE) ==\n")
 
-# Enable testing with CGAL_ENABLE_TESTING. Historically, users would enable the tests
-# by specifying BUILD_TESTING so if CGAL_ENABLE_TESTING is not enabled, we will set it
-# to the value of BUILD_TESTING
+# Enable testing with CGAL_ENABLE_TESTING. Before CGAL-6.0, users would enable
+# the tests by specifying BUILD_TESTING. For compatibility, If BUILD_TESTING is
+# set, that is the default value for CGAL_ENABLE_TESTING. Otherwise, the default
+# value is OFF.
 option(CGAL_ENABLE_TESTING "Build the testing tree." ${BUILD_TESTING})
 if(CGAL_ENABLE_TESTING AND NOT POLICY CMP0064)
   message(

Installation/CMakeLists.txt

@@ -105,8 +105,11 @@ else(CGAL_BRANCH_BUILD)
       "${CGAL_SOURCE_DIR}"
       CACHE INTERNAL "Directory containing the GraphicsView package")
 
-  # Enable testing with CGAL_ENABLE_TESTING
-  option(CGAL_ENABLE_TESTING "Build the testing tree." OFF)
+# Enable testing with CGAL_ENABLE_TESTING. Before CGAL-6.0, users would enable
+# the tests by specifying BUILD_TESTING. For compatibility, If BUILD_TESTING is
+# set, that is the default value for CGAL_ENABLE_TESTING. Otherwise, the default
+# value is OFF.
+option(CGAL_ENABLE_TESTING "Build the testing tree." ${BUILD_TESTING})
   if(CGAL_ENABLE_TESTING)
     enable_testing()
   endif()

Installation/cmake/modules/CGAL_add_test.cmake

@@ -3,7 +3,11 @@ if(CGAL_add_test_included)
 endif(CGAL_add_test_included)
 set(CGAL_add_test_included TRUE)
 
-option(CGAL_ENABLE_TESTING "Build the testing tree." OFF)
+# Enable testing with CGAL_ENABLE_TESTING. Before CGAL-6.0, users would enable
+# the tests by specifying BUILD_TESTING. For compatibility, If BUILD_TESTING is
+# set, that is the default value for CGAL_ENABLE_TESTING. Otherwise, the default
+# value is OFF.
+option(CGAL_ENABLE_TESTING "Build the testing tree." ${BUILD_TESTING})
 
 if(CGAL_ENABLE_TESTING)
   enable_testing()