Avoid explicit optimization overriding CXXFLAGS

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -620,7 +620,7 @@ if(NOT MSVC)
   set(CMAKE_CXX_FLAGS_DEBUG
       "${CMAKE_CXX_FLAGS_DEBUG} -g -O0 -DDEBUG -Wall ${M32_FLAG} ${CXX_EXTRA}")
   set(CMAKE_CXX_FLAGS_RELEASE
-      "${CMAKE_CXX_FLAGS_RELEASE} -O3 -DNDEBUG ${M32_FLAG} ${CXX_EXTRA}")
+      "${CMAKE_CXX_FLAGS_RELEASE} -DNDEBUG ${M32_FLAG} ${CXX_EXTRA}")
   if("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang$" AND CMAKE_LTO)
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -flto=${CMAKE_LTO}")
   elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_LTO)
