Do not modify cached CMAKE_CXX_FLAGS_RELEASE

Adding compiler's flags and force updating cached value
of release C++ flags lead to appending special compiler
flags on every edit of any CMakeList.txt.

For compile result this is harmless, but was annoying
slight modification of CMakeList.txt triggered full
project rebuild.

Now modified C++ flags are used for the whole subtree
starting from the project root, but this doesn't lead
to flags modified in cache.

Change-Id: Ieb32bd7f96d5a09632f0b2b5325f6567be8cb5a8

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index eee67fa..0b6e829 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -612,8 +612,7 @@
       ENDIF (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     ENDIF (${BUILD_ANDROID})
   ENDIF (CMAKE_COMPILER_IS_GNUCXX)
-  SET (CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} ${CERES_CXX_FLAGS}"
-       CACHE STRING "Release mode flags to the C++ Compiler" FORCE)
+  SET (CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} ${CERES_CXX_FLAGS}")
 ENDIF (CMAKE_BUILD_TYPE STREQUAL "Release")
 
 # After the tweaks for the compile settings, disable some warnings on MSVC.