BUG: Only test for cxx flags the first time around

Modules/TestCXXAcceptsFlag.cmake

@@ -7,6 +7,7 @@
 #
 
 MACRO(CHECK_CXX_ACCEPTS_FLAG FLAGS  VARIABLE)
+  IF(DEFINED ${VARIABLE})
     MESSAGE(STATUS "Checking to see if CXX compiler acepts flag ${FLAGS}")
     TRY_COMPILE(${VARIABLE}
       ${CMAKE_BINARY_DIR}
@@ -24,4 +25,5 @@ MACRO(CHECK_CXX_ACCEPTS_FLAG FLAGS  VARIABLE)
         "Determining if the CXX compiler accepts the flag ${FLAGS} failed with "
         "the following output:\n${OUTPUT}\n\n")
     ENDIF(${VARIABLE})
+  ENDIF(DEFINED ${VARIABLE})
 ENDMACRO(CHECK_CXX_ACCEPTS_FLAG)