Você provavelmente vai querer executar / construir todas as demos do qt abrindo os arquivos .pro para os exemplos específicos.
chown -R yourusername /opt/qt5/examples/
Quando eu fui construir o exemplo de opengl / textures abrindo o arquivo textures.pro dentro do qtcreator, ele automaticamente configurou o projeto MAS estava apontando para o QT4 por padrão.
Eu recebi um erro estranho que dizia que não encontrei o QtWidgets quando eu estava tentando construir usando o qt4. Com o Qt5 como ferramenta de construção, o problema desapareceu.
CERTIFIQUE-SE QUE PONHA NO QT5 como o projeto criado para depuração / release porque não é o padrão no qtcreator. Caso contrário, você receberá erros estranhos ao construir. Este problema é fácil de detectar na saída da compilação, que deve mostrar os caminhos qt5 include e qt5 libs.
2:58:51: Running steps for project textures...
12:58:51: Starting: "/usr/bin/make" clean -w
make: Entering directory '/opt/qt5/examples/qtbase/opengl/textures-build-Desktop Release'
rm -f qrc_textures.cpp
rm -f moc_glwidget.cpp moc_window.cpp
rm -f glwidget.o main.o window.o qrc_textures.o moc_glwidget.o moc_window.o
rm -f *~ core *.core
make: Leaving directory '/opt/qt5/examples/qtbase/opengl/textures-build-Desktop-Release'
12:58:52: The process "/usr/bin/make" exited normally.
12:58:52: Configuration unchanged, skipping qmake step.
12:58:52: Starting: "/usr/bin/make" -w
make: Entering directory '/opt/qt5/examples/qtbase/opengl/textures-build-Desktop-Release'
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o glwidget.o ../textures/glwidget.cpp
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o main.o ../textures/main.cpp
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o window.o ../textures/window.cpp
/opt/qt5/bin/rcc -name textures ../textures/textures.qrc -o qrc_textures.cpp
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o qrc_textures.o qrc_textures.cpp
/opt/qt5/bin/moc -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. ../textures/glwidget.h -o moc_glwidget.cpp
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o moc_glwidget.o moc_glwidget.cpp
/opt/qt5/bin/moc -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. ../textures/window.h -o moc_window.cpp
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -fPIE -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../../../../mkspecs/linux-g++-64 -I../textures -I../../../../include -I../../../../include/QtOpenGL -I../../../../include/QtWidgets -I../../../../include/QtGui -I../../../../include/QtCore -I. -I. -o moc_window.o moc_window.cpp
g++ -m64 -Wl,-O1 -Wl,-rpath,/opt/qt5/lib -o textures glwidget.o main.o window.o qrc_textures.o moc_glwidget.o moc_window.o -L/usr/X11R6/lib64 -L/opt/qt5/lib -lQtOpenGL -lQtWidgets -lQtGui -lQtCore -lGL -lpthread
make: Leaving directory '/opt/qt5/examples/qtbase/opengl/textures-build-Desktop-Release'
12:58:57: The process "/usr/bin/make" exited normally.