Add windows compiler flag to not treat wchar as built in.

This is required for the QString::fromStdWString type methods to work and
seems to have no ill effect.
Refs #9252

Code/Mantid/Build/CMake/WindowsSetup.cmake

@@ -29,7 +29,9 @@ endif()
 #           as a level 3 warning
 # /w34389 - Treat warning C4389, about equality comparison on unsigned 
 #           and signed, as a level 3 warning
-set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP /w34296 /w34389" )
+# /Zc:wchar_t- - Do not treat wchar_t as a builtin type. Required for Qt to
+#           work with wstring
+set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP /w34296 /w34389 /Zc:wchar_t-" )
 # As discussed here: http://code.google.com/p/googletest/issues/detail?id=412
 # gtest requires changing the _VARAIDIC_MAX value for VS2012 as it defaults to 5
 if ( MSVC_VERSION EQUAL 1700 )