When the qt install prefix and the qt host data path are the same,
CMake's RELATIVE_PATH file path function will return the empty string.
This made us accidentally set ECM_MKSPECS_INSTALL_DIR to /mkspecs/modules.
Fix this by explicitly checking for the empty string. Once we can depend on
CMake 3.20, we can use CMake's builtin function for joining paths instead.

When building multiple KDE projects, installing them to "$DESTDIR"
and having cmake look for kde dependencies in "$DESTDIR" using
"CMAKE_PREFIX_PATH=$DESTDIR/usr", we currently get failures when
calling find_package() on projects that use KDE_INSTALL_QTPLUGINDIR,
KDE_INSTALL_QTQMLDIR and KDE_INSTALL_QTQUICKIMPORTSDIR because these
are defined as absolute paths when KDE_INSTALL_USE_QT_SYS_PATHS is
enabled.

This commit defines these paths relative to qmake's QT_INSTALL_PREFIX
property instead when KDE is install to the same prefix as Qt. This
fixes DESTDIR installations because with relative paths, CMake will
search for these paths in "$DESTDIR" as well as "/".

We limit this change to the scenario where the Qt and CMake install
prefixes are the same because always doing this would break backwards
compatibility as qml and plugins would be installed into CMAKE_INSTALL_PREFIX
instead of QT_INSTALL_PREFIX after this change.

GIT_SILENT

Ensures that the files
* are re-created by build rule when accidentally deleted in the build dir
* are not getting new timestamps on every cmake run, even when content
  has not changed

COLS_IN_ALPHA_INDEX got obsolete in doxygen 1.9.0, besides not being
used anyway due to ALPHABETICAL_INDEX being set to NO.

Also remove IGNORE_PREFIX, unused as well due to ALPHABETICAL_INDEX=NO

NO_CHANGELOG