Do not unset cache variables in KDEInstallDirs.

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