Cache variables such as CMAKE_INSTALL_BINDIR may be used by other
modules included from parallel parts of the tree, so we should not touch
them. We still override them in the runtime environment, but this will
not interfere with parallel subtrees of the project.

As part of this, the order of precedence of variables specified on the
command line is changed, so that KDE_INSTALL_* is considered first
(although it is still considered "undefined" in the documentation). This
means that if you only specify CMAKE_INSTALL_BINDIR, that will be used
by both KDEInstallDirs and GNUInstallDirs, but if you specify both that
and KDE_INSTALL_BINDIR, KDEInstallDirs will use KDE_INSTALL_BINDIR
instead.  This is probably not too useful, but seems more useful than
any other arrangement.

BUG: 342717
REVIEW: 121982
(cherry picked from commit 4462e9ddedb78c31a6f1089d073b1bfde6ce60ce)

(cherry picked from commit d47ef4776d349110988477ea1836d98a9e393755)

A previous commit inadvertantly changed the KF5 include prefix to kf5
(uppercase to lowercase).

REVIEW: 121646

Creating variables whose names start with CMAKE_ is a bad idea for
modules distributed outside CMake itself. Since the module is called
KDEInstallDirs, having a KDE_INSTALL_ prefix for the variables is clear
and intuitive.

Both CMAKE_INSTALL_* variables and the older KDELibs4-compatible
variables are provided, unless KDE_INSTALL_DIRS_NO_DEPRECATED is set to
TRUE before the module is included. Even then, the CMAKE_INSTALL_*
variables provided by the GNUInstallDirs module will still be set and
understood (for compatibility with that module), unless
KDE_INSTALL_DIRS_NO_CMAKE_VARIABLES is set to TRUE.

REVIEW: 121646

Having done a lot of (unrelated) Windows development recently, I've come
to understand the situation much better, so I'm recording that
information here for the benefit of others.

When installing to /usr, we should use /etc for configuration. Using
/usr/etc does not make sense.

REVIEW: 120246

when running with the KDE_INSTALL_USE_QT_SYS_PATHS option allow QMLDIR in
KDEInstallDirs to follow whatever is defined by qmake

this change makes sure that qml plugins will end up in a default Qt path
when using the super special magic flag.

KDE modules cannot assume the normal ECM modules are in the CMake module
path, and CMAKE_INSTALL_IMPORTS_INSTALL_DIR / QTQUICKIMPORTSDIR was not
set correctly. Also, ECMQueryQmake.cmake used a deprecated CMake command
(exec_program).