Собираю сейчас Qt 5.4.2 под Win32 c mingw-builds 4.9.1. Процесс у меня прервался (мля, ну почему это всегда крайне редко работает сразу?), но Qt5Core собралось. Так вот, там нет зависимости от ICU.
dumpbin /dependents Qt5Core.dll:
Microsoft (R) COFF/PE Dumper Version 10.00.40219.01
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file Qt5Core.dll
File Type: DLL
Image has the following dependencies:
ADVAPI32.dll
libgcc_s_dw2-1.dll
KERNEL32.dll
MPR.DLL
msvcrt.dll
ole32.dll
libwinpthread-1.dll
SHELL32.dll
USER32.dll
WS2_32.dll
libstdc++-6.dll
Summary
1000 .CRT
4000 .bss
2000 .data
3F000 .edata
72000 .eh_fram
3000 .idata
19D000 .rdata
13000 .reloc
1000 .rsrc
2AD000 .text
1000 .tls
Похоже, что только WebKit зависит от ICU. Но, если не ошибаюсь, в configure есть возможность управлять зависимостью от ICU... Да, и вот
http://doc.qt.io/qt-5.4/windows-requirements.html:
Qt WebKit, and applications using it, always need the ICU libraries. Qt Core can be configured to either rely on ICU functionality, or fall back to Windows API and internal algorithms with somewhat reduced functionality.
From Qt 5.3 and onwards, configure does not link Qt Core against ICU libraries anymore by default. This reduces the size of a self-contained application package considerably, as long as Qt WebKit is not used.
Letting Qt Core utilize the ICU libraries however has following advantages:
Behavior matches other platforms more closely.
Extended set of text codecs (see QTextCodec).
QLocale::toUpper(), QLocale::toLower() always use case conversion rules specific to the locale.
QCollator::setNumericMode() does work consistently on all Windows versions.
To explicitly enable the use of ICU in Qt Core, pass -icu to configure:
configure -icu
Удачи.