VS6: Avoid SBCS test on VS6 (#12189)

The previous commit, df19b9ca, assumed that the corresponding
_UNICODE functionality in the VS6 generator actually worked.
That turns out not to be the case. Unicode definition does not
actually suppress _MBCS definition, so neither does this new
code...

Don't test it here, since the patch submitter for _SBCS does
not need VS6 support.

If somebody needs _UNICODE and _SBCS support to suppress the
definition of _MBCS in the VS6 generator, it is work yet to do.
Patches welcome.

Tests/CMakeLists.txt

@@ -1306,7 +1306,9 @@ ${CMake_BINARY_DIR}/bin/cmake -DVERSION=master -P ${CMake_SOURCE_DIR}/Utilities/
   endif()
 
   IF(${CMAKE_TEST_GENERATOR} MATCHES "Visual Studio")
-    ADD_TEST_MACRO(SBCS SBCS)
+    IF(NOT MSVC60)
+      ADD_TEST_MACRO(SBCS SBCS)
+    ENDIF(NOT MSVC60)
 
     ADD_TEST(VSExternalInclude ${CMAKE_CTEST_COMMAND}
       --build-and-test