Fixed problem on archlinux when linking and undefined reference to clock_gettime on ArchLinux, CMake Caused an error when CMAKE_BUILD_TYPE is empty

CMakeLists.txt

@@ -6,7 +6,7 @@ set(GAMEPLAY_VERSION 1.5.0)
 
 # debug
 message( "CMAKE_BUILD_TYPE: ${CMAKE_BUILD_TYPE}" )
-if( ${CMAKE_BUILD_TYPE} STREQUAL "DEBUG" )
+if( "${CMAKE_BUILD_TYPE}" STREQUAL "DEBUG" )
     add_definitions(-D_DEBUG)
 endif()