Modules/Test*.cmake: Use if(DEFINED) to simplify conditions

Replace old hacks of the form 'if("${VAR}" MATCHES "^${VAR}$")'
with the much simpler 'if(NOT DEFINED ${VAR})'.

Modules/TestForSTDNamespace.cmake

@@ -23,7 +23,7 @@
 # (To distribute this file outside of CMake, substitute the full
 #  License text for the above reference.)
 
-if("CMAKE_STD_NAMESPACE" MATCHES "^CMAKE_STD_NAMESPACE$")
+if(NOT DEFINED CMAKE_STD_NAMESPACE)
   message(STATUS "Check for STD namespace")
   try_compile(CMAKE_STD_NAMESPACE  ${CMAKE_BINARY_DIR}
     ${CMAKE_ROOT}/Modules/TestForSTDNamespace.cxx