control flag options between MSVC and MINGW

dev/externals/reverb/CMakeLists.txt

@@ -3,11 +3,12 @@ if(APPLE)
   set(CMAKE_CXX_FLAGS "-O2 -Wall   -Dunix -fomit-frame-pointer -funroll-loops -std=c++11 -stdlib=libc++")
 else()
   if(MINGW OR MSVC)
-    set(CMAKE_C_FLAGS "-O3 -DWIN32 -D_WIN32 -fomit-frame-pointer  -funroll-loops")
     if(MINGW)
-        set(CMAKE_CXX_FLAGS "-O3 -DWIN32 -D_WIN32 -fomit-frame-pointer  -funroll-loops -static-libgcc -static-libstdc++")
+        set(CMAKE_C_FLAGS "-O2 -DWIN32 -D_WIN32 -fomit-frame-pointer  -funroll-loops")
+        set(CMAKE_CXX_FLAGS "-O2 -DWIN32 -D_WIN32 -fomit-frame-pointer  -funroll-loops -static-libgcc -static-libstdc++")
     else()
-        set(CMAKE_CXX_FLAGS "-O3 -DWIN32 -D_WIN32 -fomit-frame-pointer  -funroll-loops")
+        set(CMAKE_C_FLAGS "-O2 -DWIN32 -D_WIN32")
+        set(CMAKE_CXX_FLAGS "-O2 -DWIN32 -D_WIN32")
     endif()
   else()
     set(CMAKE_C_FLAGS "-O3 -Wall -Dlinux -Dunix -fomit-frame-pointer -funroll-loops")