[cmake] fix another whitespace/list/string snafu

CMakeLists.txt

@@ -51,8 +51,9 @@ execute_process(
 )
 
 SET( DS_FLAGS_Release CACHE STRING
-  "-O2 -fPIC ${CMAKE_CXX_FLAGS_RELEASE}")
-set(DS_FLAGS_RelWithDebInfo "-g ${DS_FLAGS_Release}")
+  "-O2" "-fPIC" "${CMAKE_CXX_FLAGS_RELEASE}")
+set(DS_FLAGS_RelWithDebInfo
+  "-g" "${DS_FLAGS_Release}")
 
 SET( DS_FLAGS_Debug CACHE STRING
   "-O0 -g3 -ggdb -Wunused-variable -fno-strict-aliasing -fPIC -Wall -Wextra -Wparentheses -pedantic  -Wshadow -Wundef -Wnon-virtual-dtor")