Move all compiler/language features, like -std=c++11 or
-fsanitize=address for ASAN builds,
from CMAKE_<LANG>_FLAGS_<CONFIG> to CMAKE_<LANG>_FLAGS

With this patch, CMAKE_<LANG>_FLAGS_DEBUG contains only debug related flags:
-- CMAKE_CXX_FLAGS_DEBUG: -DSAFE_MUTEX -DENABLED_DEBUG_SYNC -g

The other CMAKE_<LANG>_FLAGS_<CONFIG> flags will switch off debugging,
and add our default optimization flags:
-- CMAKE_CXX_FLAGS_MINSIZEREL: -DDBUG_OFF -ffunction-sections -fdata-sections -Os -DNDEBUG

Always PREPEND MySQL default flag values to cmake compiler flags,
so that they can easily be overriden by command line options.
This fixes
Bug#28208842 COMPILER OPTIMIZATION HARDWIRED TO O2

To override MySQL default flag values, for e.g. RELEASE builds, do:
cmake . -DCMAKE_CXX_FLAGS_RELEASE="-O3 -DNDEBUG -fno-function-sections -fno-data-sections"

Change-Id: Ia7786b017fb719c203f4be0e83cf5f9e18b318dc
(cherry picked from commit 3a8897a97a29d8b8ed042e24dbe1b1143a226582)