Cannot (re)build the debugging helper

After installing the final release of Qt SDK 1.1 on Ubuntu 11.04, and updating my PATH, I started Qt Creator and tried to (re)build the debugging helper for /opt/QtSDK/Desktop/Qt/473/gcc/bin/qmake. However, I did get the following error message:

Error running '/usr/bin/make' in /home/alan/.local/share/data/Nokia/QtCreator/qtc-qmlobserver/116831519/: The process returned exit code 2:
/opt/QtSDK/Desktop/Qt/473/gcc/bin/uic recopts.ui -o ui_recopts.h
/opt/QtSDK/Desktop/Qt/473/gcc/bin/uic proxysettings.ui -o ui_proxysettings.h
/opt/QtSDK/Desktop/Qt/473/gcc/bin/moc -DGL_SUPPORTED -DNO_PRIVATE_HEADERS -DBUILD_QMLJSDEBUGGER_STATIC_LIB -DQWEAKPOINTER_ENABLE_ARROW -DQT_DECLARATIVE_LIB -DQT_SCRIPT_LIB -DQT_SQL_LIB -DQT_OPENGL_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I/opt/QtSDK/Desktop/Qt/473/gcc/mkspecs/linux-g++ -I. -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtCore -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtNetwork -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtGui -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtOpenGL -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtSql -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtScript -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtDeclarative -I/opt/QtSDK/Desktop/Qt/473/gcc/include -I. -Iqmljsdebugger/include -Iqmljsdebugger/editor -I/usr/X11R6/include -I. -I. qmlruntime.cpp -o qmlruntime.moc
g++ -c -pipe -g -Wall -W -D_REENTRANT -DGL_SUPPORTED -DNO_PRIVATE_HEADERS -DBUILD_QMLJSDEBUGGER_STATIC_LIB -DQWEAKPOINTER_ENABLE_ARROW -DQT_DECLARATIVE_LIB -DQT_SCRIPT_LIB -DQT_SQL_LIB -DQT_OPENGL_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I/opt/QtSDK/Desktop/Qt/473/gcc/mkspecs/linux-g++ -I. -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtCore -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtNetwork -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtGui -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtOpenGL -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtSql -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtScript -I/opt/QtSDK/Desktop/Qt/473/gcc/include/QtDeclarative -I/opt/QtSDK/Desktop/Qt/473/gcc/include -I. -Iqmljsdebugger/include -Iqmljsdebugger/editor -I/usr/X11R6/include -I. -I. -o qmlruntime.o qmlruntime.cpp
In file included from /opt/QtSDK/Desktop/Qt/473/gcc/include/QtOpenGL/QGLWidget:1:0,
from qmlruntime.cpp:105:
/opt/QtSDK/Desktop/Qt/473/gcc/include/QtOpenGL/qgl.h:77:20: fatal error: GL/gl.h: No such file or directory
compilation terminated.
make: *** [qmlruntime.o] Error 1