[cmake] make CUSTOM_FLAGS string again

cmake/DuneUtils.cmake

@@ -122,7 +122,7 @@ CHECK_INCLUDE_FILE_CXX("tr1/array" HAVE_TR1_ARRAY)
 CHECK_INCLUDE_FILE_CXX("malloc.h" HAVE_MALLOC_H)
 
 SET( CUSTOM_FLAGS
-	-Wall -Wextra -pedantic -O3 -DDNDEBUG -funroll-loops -g -ggdb -DENABLE_ADAPTIVE=1 -DADAPTIVE_SOLVER -DUSE_BFG_CG_SCHEME -fno-strict-aliasing CACHE LIST  
+	"-Wall -Wextra -pedantic -O3 -DDNDEBUG -funroll-loops -g -ggdb -DENABLE_ADAPTIVE=1 -DADAPTIVE_SOLVER -DUSE_BFG_CG_SCHEME -fno-strict-aliasing" CACHE STRING  
 	"CUSTOM FLAGS")
 FIND_PACKAGE(Boost 1.35.0 REQUIRED)
 INCLUDE_DIR(${Boost_INCLUDE_DIR})