Windows: Do not link QtCore against ICU by default

Do not link against ICU on Windows, unless it is explicitly requested
('-icu' configure option). This removes the QtCore->ICU dependency
if ICU was detected in the configure environment.

So far ICU has been used in Qt Core for

- support of a larger set of codecs (805 instead of 135)
- more accurate QLocale functionality (QLocale::toUpper,
  qLocale::toLower)
- string collation

However, for all functionality there are also backends using the
Windows API/Registry (QLocale, QCollator), or built-in
codecs that are part of QtCore. Since the ICU dependency is quite heavy
(3 libs with about 25 MB) it seems sensible to not require it by default.

QtWebkit is unaffected, since it has it's own ICU check.

[ChangeLog][Windows] Changed configure defaults so that Qt5Core does not
link against ICU libraries anymore. Pass '-icu' to enable it.

Task-number: QTBUG-38259
Change-Id: I3fd3e8ac0f091f532b04945718c0e4a3cc71a087
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

configure

@@ -1331,6 +1331,7 @@ while [ "$#" -gt 0 ]; do
             XPLATFORM_MINGW=yes
             CFG_RPATH=no
             CFG_REDUCE_EXPORTS=no
+            CFG_ICU=no
             ;;
         esac
         ;;

tools/configure/configureapp.cpp

@@ -258,7 +258,7 @@ Configure::Configure(int& argc, char** argv)
 
     dictionary[ "PCRE" ]            = "auto";
 
-    dictionary[ "ICU" ]             = "auto";
+    dictionary[ "ICU" ]             = "no";
 
     dictionary[ "ANGLE" ]           = "auto";
     dictionary[ "DYNAMICGL" ]       = "auto";