Add support for using ICU DLL from Windows 10

Requires Windows 10 version 1903 (May 2019 Update):
https://docs.microsoft.com/en-us/windows/win32/intl/international-components-for-unicode--icu-

Also adds missing configure check for unicode/utext.h.

Source/Additions/Unicode.m

@@ -56,6 +56,9 @@
 #if     defined(HAVE_UNICODE_UCNV_H)
 #include <unicode/ucnv.h>
 #endif
+#if     defined(HAVE_ICU_H)
+#include <icu.h>
+#endif
 
 
 typedef struct {unichar from; unsigned char to;} _ucc_;