Core/Buildsystem: Clean up compileflags when compiling with warnings fully enabled.

-Wwrite-strings is default enabled when -Wall is set in clang/gcc, hence removed.
 -Woverloaded-virtual is now enabled for C++-code only, this removes the useless warnings when dabbling with C-only code.

CMakeLists.txt

@@ -12,7 +12,7 @@
 project(TrinityCore)
 
 # CMake policies (can not be handled elsewhere)
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 2.8)
 cmake_policy(SET CMP0005 OLD)
 
 # Set RPATH-handing (CMake parameters)