In CMake, variables like CMAKE_CXX_FLAGS_RELEASE etc should be cache
variables, so users can set them from outside. ROOT, however, FORCE
overwrote them, so users cannot change anything.
Now, the variables are not set by ROOT at all, we use the CMake
defaults.
It is fine to append or replace substrings, but the variables should NOT
be overwritten to give users some options.

In case ROOT wants to move away from CMake defaults, it can be done like
this:
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e40b84a920..ab41612006 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -29,6 +29,11 @@ endforeach()

 include(cmake/modules/CaptureCommandLine.cmake)

+set(CMAKE_CXX_FLAGS_DEBUG_INIT "-O1 -g")
+set(CMAKE_CXX_FLAGS_ASSERT_INIT "-O2 -g")
+
 project(ROOT)