On Windows the Shlwapi.h file has a macro:

  #define StrCat lstrcatA

And benchmark/src/string_util.h defines StrCat and it is renamed to
lstrcatA if we don't undef the macro in Shlwapi.h. This is an innocuous
bug if string_util.h is included after Shlwapi.h, but it is a compile
error if string_util.h is included before Shlwapi.h.

This fixes issue #545.