Clean up optimization flags

* The way -O3 was set for release builds was accidentally removing
  -DNDEBUG
* -Og seems to mess with debugging with lldb, even though the GCC manual
  page says to use it for debug builds, so remove it

CMakeLists.txt

@@ -73,10 +73,8 @@ if (ENABLE_LTO)
     set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif()
 
-set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Og")
-set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -Og")
-set(CMAKE_C_FLAGS_RELEASE "-O3")
-set(CMAKE_CXX_FLAGS_RELEASE "-O3")
+string(REPLACE "-O2" "-O3" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
+string(REPLACE "-O2" "-O3" CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
 
 if (NOT APPLE)
     set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS_RELEASE} -s")