Passing absolute paths to CMake causes errors

Hey,

In order to check the changes we'll need to do to our packages, I tried building an experimental Qt6 qtbase package for openSUSE, using the wip/cmake branch.

We pass absolute paths to CMake[1]. The build system doesn't behave as expected in these conditions.

1/ syncqt is lost when using absolute paths. The generated private headers are created in $builddir/usr/include/qt6/<module>/<version> which is expected in this configuration, but others are created in $builddir/include/<module>/, causing build errors

2/ The build system doesn't use the INSTALL_* variables in a couple places (eg for installing binaries)

3/ The generated CMake config targets contain wrong path (eg: INTERFACE_INCLUDE_DIRECTORIES is hardcoded to `${_IMPORT_PREFIX}/include/QtCore`)

4/ qmake -query returns wrong paths

5/ Enabling BUILD_TESTING failed with our command line

[1] The command line used is cmake -DCMAKE_INSTALL_PREFIX=/usr -DINSTALL_ARCHDATADIR=/usr/lib64/qt6 -DINSTALL_BINDIR=/usr/lib64/qt6/bin -DINSTALL_DATADIR=/usr/share/qt6 -DINSTALL_DOCDIR=/usr/share/doc/packages/qt6 -DINSTALL_EXAMPLESDIR=/usr/lib64/qt6/examples -DINSTALL_INCLUDEDIR=/usr/include/qt6 -DINSTALL_LIBDIR=/usr/lib64 -DINSTALL_LIBEXECDIR=/usr/lib64/qt6/libexec -DINSTALL_MKSPECSDIR=/usr/lib64/qt6/mkspecs -DINSTALL_PLUGINSDIR=/usr/lib64/qt6/plugins -DINSTALL_QMLDIR=/usr/lib64/qt6/qml -DINSTALL_SYSCONFDIR=/usr/etc/xdg -DINSTALL_TRANSLATIONSDIR=/usr/share/qt6/translations -DBUILD_EXAMPLES=FALSE -DBUILD_TESTING=FALSE -DBUILD_WITH_PCH=FALSE