Use a relative install_name for QtWebEngineProcess' dependencies

Until we can rely on Qt using @rpath on OSX, QtWebEngineProcess'
dependencies must either be updated by macdeployqt when creating
an .app bundle, either we must make that path relative at build
time.

Since QtWebEngineProcess.app is always deployed inside
QtWebEngineCore.framework, we can safely assume that the dylib is
always in a parent directory and that QtCore.framework can be found
one directory higher.

The additional dependencies of QtWebEngineCore.framework will be
handled by the dynamic linker when it gets loaded, but this only
works for QtWebEngineProcess when macdeployqt uses @loader_path
instead of @executable_path (then relative to the application which
is in a different directory than QtWebEngineProcess).
This can be forced by passing a non-empty value for its
-executable= command-line argument (e.g
`macdeployqt Browser.app -executable=Browser.app/Contents/MacOS/Browser`)

All this will be much simpler once we got Qt framework builds to use
@rpath instead in a future minor version.

Task-number: QTBUG-41611
Change-Id: Ied46c65a09f7033b622708da6e3d85855f9abf34
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Disable using the ICU data file on embedded linux

Since we use the system ICU we do not need the extra data file.
This fixes a crash caused by missing icudtl.dat file.

Change-Id: Ic7a6ad83d6595ea3a89c736c281499636b05945e
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>