[Development] Setting CMAKE_PREFIX_PATH to Qt prefix breaks use of system ICU via CMake

[Konstantin Tokarev]

Hello,

There is a strange situation involving official Qt SDK (>=5.8.0) binaries for Linux, ICU, cmake, and WebKit project files, I'm not sure which side really needs to be fixed.

(Qt)WebKit uses custom module to find ICU, you can see its code at [1]. Module uses quite popular practise of invoking pkg-config first, and then performing search of libs and headers by passing them as HINTS to find_path() and find_library(). Doing so allows to have a meaningful fallback if pkg-config is missing, misconfigured, or e.g. returns host libs when cross-compilation is needed.

Before Qt 5.8.0, Qt SDK didn't include unversioned symlinks for ICU libraries, e.g. 5.7 had only libicuuc.so.56 and libicuuc.so.56.1, but not libicuuc.so. Starting from 5.8.0, unversioned symlinks are also provided.

Now, if someone sets CMAKE_PREFIX_PATH to the Qt root in SDK, cmake starts to find ICU libraries in SDK, because  priority logic of find_library makes CMAKE_PREFIX_PATH preferred to the HINTS provided by pkg-config.

At the same time, there are no ICU headers in Qt SDK, so include paths are taken from pkg-config, that detects system-wide installation of ICU. Note that ICU uses versioned suffixes in symbol names, so different versions are binary incompatible, resulting in the broken build when headers don't match libraries.

There are a few possible solutions:

* Provide ICU headers as a part of Qt SDK too. This has a benefit that ICU libraries, required for QtCore, can be reused in other projects that use Qt and ICU, e.g. for building of experimental QtWebKit versions against binary SDK.
* Remove unversioned symlinks like libicuuc.so from SDK, so that they are not found by FindICU.cmake, and also by linker if it's given -licuuc
* Avoid using CMAKE_PREFIX_PATH with Qt SDK, using Qt5_DIR and similar variables instead
* Change FindICU to ignore libraries Qt-related paths - ugly hack, also FindICU.cmake is technically 3rd party code for QtWebKit
* Change priorities in CMake's find_library - probably off-topic for this list, and may be impossible because of backward compatibility reasons.

Thoughts?

[1] https://github.com/annulen/webkit/blob/qtwebkit-stable/Source/cmake/FindICU.cmake

-- 
Regards,
Konstantin