ENH: fix bug, write the correct file

Modules/CheckCXXSourceCompiles.cmake

@@ -20,7 +20,7 @@ MACRO(CHECK_CXX_SOURCE_COMPILES SOURCE VAR)
       SET(CHECK_CXX_SOURCE_COMPILES_ADD_INCLUDES
         "-DINCLUDE_DIRECTORIES:STRING=${CMAKE_REQUIRED_INCLUDES}")
     ENDIF(CMAKE_REQUIRED_INCLUDES)
-    FILE(WRITE "${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/src.c"
+    FILE(WRITE "${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/src.cxx"
       "${SOURCE}")
 
     MESSAGE(STATUS "Performing Test ${VAR}")