BUG: fix test not to run every time

Modules/TestForSTDNamespace.cmake

@@ -3,8 +3,7 @@
 #
 # CMAKE_NO_STD_NAMESPACE - defined accoreding to the results
 #
-
-IF(NOT CMAKE_STD_NAMESPACE)
+IF("CMAKE_STD_NAMESPACE" MATCHES "^CMAKE_STD_NAMESPACE$")
   MESSAGE(STATUS "Check for STD namespace")
   TRY_COMPILE(CMAKE_STD_NAMESPACE  ${CMAKE_BINARY_DIR} 
     ${CMAKE_ROOT}/Modules/TestForSTDNamespace.cxx)
@@ -17,7 +16,8 @@ IF(NOT CMAKE_STD_NAMESPACE)
     SET (CMAKE_NO_STD_NAMESPACE 1 CACHE INTERNAL 
        "Does the compiler support std::.")
   ENDIF (CMAKE_STD_NAMESPACE)
-ENDIF(NOT CMAKE_STD_NAMESPACE)
+ENDIF("CMAKE_STD_NAMESPACE" MATCHES "^CMAKE_STD_NAMESPACE$")
+