cmake: set -O2 optimization level only if CMAKE_BUILD_TYPE is not specified

CMakeLists.txt

@@ -487,7 +487,7 @@ source_group (Predictor\\Library FILES ${LIBPREDICTOR})
 source_group (Parser FILES ${PARSER})
 source_group (PPPA\\PPPA FILES ${PPPA})
 source_group (PPPA\\ZLib FILES ${ZLIB})
-   
+
 if (MSVC_IDE)   
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP /Zc:__cplusplus")
 else()
@@ -496,7 +496,9 @@ else()
     else()
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
     endif()
-   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2")
+    if(NOT CMAKE_BUILD_TYPE)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2")
+    endif()
 endif()
 
 add_subdirectory(projects/Fdvm)