[Qt] Fix build when cross-compiling

[Qt] Fix build when cross-compiling
https://bugs.webkit.org/show_bug.cgi?id=77634

Patch by Simon Hausmann <simon.hausmann@nokia.com> on 2012-02-03
Reviewed by Tor Arne Vestbø.

Source/WebKit2:

* WebKit2.pri: Link in -lrt on linux mkspecs, regardless of whether gcc
is in use or not. More specifically this fixes the case where the makespace
is not linux-g++* but for a cross-compiling one like linux-arm-gnueabi-g++.

Source/WTF:

* WTF.pri: Don't use icu-config for determining the library search
path and libraries needed for ICU. Either we use icu-config for includes
_and_ libraries or we don't. Right now we assume that icu is a system library
and expect the headers in the default include search pathes (/usr/include for example).
However we use icu-config to figure out where the libraries are, which breaks when
cross-compiling, because icu-config isn't cross-compile friendly (I wish icu was using
pkg-config). I think for the time being we should require ICU as a _system_ library,
which implies the header and library availability in default search paths. This also
makes the build succeed when cross-compiling with --sysroot.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@106651 268f45cc-cd09-0410-ab3c-d52691b4dbfc