Qt – Make qmake use qt5 by default

sudo apt-get install qt5-default works for me.

$ aptitude show qt5-default

This package sets Qt 5 to be the default Qt version to be used when using development binaries like qmake. It provides a default configuration for qtchooser, but does not prevent alternative Qt installations from being used.

I found two ways to suppress warnings from Qt's headers, one way by installing Qt in system's path (as suggested in the other answer) and the other directly from your pro file by using GCC flags.

1. When building your own Qt, configure the header's installation path to one of your system path:

   $ ./configure -headerdir /usr/local/include
   

   System paths are /usr/include or /usr/local/include or one of the rest listed in:

   $ grep DEFAULT_INCDIRS mkspecs/qconfig.pri
   QMAKE_DEFAULT_INCDIRS = /usr/include/c++/4.8 /usr/include/x86_64-linux-gnu/c++/4.8 /usr/include/c++/4.8/backward /usr/lib/gcc/x86_64-linux-gnu/4.8/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed /usr/include/x86_64-linux-gnu /usr/include
   

   Source: this thread in Qt's devel list.

2. Or directly in your Qt pro file, simply add the -isystem flag into the QMAKE_CXXFLAGS:

   # the line below suppresses warnings generated by Qt's header files: we tell
   # GCC to treat Qt's headers as "system headers" with the -isystem flag
   QMAKE_CXXFLAGS += -isystem $$[QT_INSTALL_HEADERS]
   

   The resulting GCC command line looks like:

   g++ -c -pipe -isystem /usr/local/Qt-5.4.1/include -Wall ...
       -I/usr/local/Qt-5.4.1/include
       -I/usr/local/Qt-5.4.1/include/QtWidgets
       ...
   

   Note how the Qt's include paths are still added with -I, allowing Qt Creator to "see" all Qt headers, but GCC sees the -isystem flag and suppresses warnings for all subfolders.