I've seen that myself but now the first user asks about it:

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Hi,

While using Qt 4.4-tp1 I noticed the following interesting little thing on
Linux. I compile my little app that uses Phonon, Taglib and some other small
features and noticed that -pthread is present on the link line "a few" times:

g++ Wl,-no-undefined -Wl,-rpath,/opt/qt-4.4-tp1/lib
-o sm <loads of object files>
-L/opt/qt-4.4-tp1/lib -llirc_client -ltag -lphonon -L/opt/qt-4.4-tp1/lib
-lQtDBus -pthread -pthread -pthread -pthread -pthread -pthread -ldbus-1 -lQtXml
-pthread -pthread -pthread -pthread -pthread -pthread -pthread -pthread -L/usr/X11R6/lib
-pthread -pthread -pthread -pthread -pthread -pthread -lQtSql -pthread -pthread -lQtGui -lpng
-lSM -lICE -pthread -pthread -lXrender -lXrandr -lfreetype -lfontconfig -lXext -lX11 -lQtNetwork
-pthread -pthread -lQtCore -lz -lm -pthread -lgthread-2.0 -lrt -lglib-2.0 -ldl -lpthread

The .pro file contains nothing magical and apart from:

TEMPLATE = app
TARGET = sm
DEPENDPATH += .