lib/icu/Compare: add Windows implementation

Using CompareStringEx() and FindNLSStringEx().

Implements a missing piece for
https://github.com/MusicPlayerDaemon/MPD/issues/820

src/lib/icu/Compare.hxx

@@ -23,13 +23,23 @@
 #include "util/Compiler.h"
 #include "util/AllocatedString.hxx"
 
+#ifdef _WIN32
+#include <wchar.h>
+#endif
+
 /**
  * This class can compare one string ("needle") with lots of other
  * strings ("haystacks") efficiently, ignoring case.  With some
  * configurations, it can prepare a case-folded version of the needle.
  */
 class IcuCompare {
+#ifdef _WIN32
+	/* Windows API functions work with wchar_t strings, so let's
+	   cache the MultiByteToWideChar() result for performance */
+	AllocatedString<wchar_t> needle;
+#else
 	AllocatedString<> needle;
+#endif
 
 public:
 	IcuCompare():needle(nullptr) {}