[Cmake] CMAKE_INSTALL_PREFIX issue

I want to setup a default value for CMAKE_INSTALL_PREFIX
if the user did not indicate a value.

so I put:

IF (NOT CMAKE_INSTALL_PREFIX)
  SET(CMAKE_INSTALL_PREFIX "/opt/${PACKAGE_NAME}-${PACKAGE_VERSION}"
CACHE PATH "Install path prefix" FORCE)
ELSE (NOT CMAKE_INSTALL_PREFIX)
  MESSAGE("CMAKE_INSTALL_PREFIX already set to: ${CMAKE_INSTALL_PREFIX}")
ENDIF (NOT CMAKE_INSTALL_PREFIX)

but is seems that CMAKE_INSTALL_PREFIX is always set in cache
BEFORE I test this :((

What is the good way to change the default value of CMAKE_INSTALL_PREFIX
besides the usual:

cmake -DCMAKE_INSTALL_PREFIX=/opt/ ../mysource

I wanted the cmake run as simple as possible like

cmake /path/to/source

--------

This question has come up several times and I've been deferring a fix
until a project-specified initial cache feature is implemented.  That
feature has been delayed because it is tricky.

I've just added a fix to CVS CMake for this.  It will be in 2.4.4 (the
upcoming patch release).  Basically your code needs to know whether the
current value of CMAKE_INSTALL_PREFIX was set by the user on the command
line or initialized by CMake.  Now when CMake initializes the variable
it also sets CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT to true.  This
enables projects to change the default install prefix like this:

IF(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
  SET(CMAKE_INSTALL_PREFIX
    "/opt/foo" CACHE PATH "FOO install prefix" FORCE
    )
ENDIF(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)

----

> Does it do this for all internal cmake variables or only for CMAKE_INSTALL_PREFIX ?

Just the install prefix.  I don't want to do it for every variable.  For
that we need a PreLoad.cmake feature (which is partially implemented but
not really working) to have project-specific initial cache entries.  I
just put this one in as a hack because so many users have asked for this
specific capability and it is a simple change before the 2.4.4 release.

-Brad

-----

Try dropping the Boolean logic and FORCE.  i.e., use just

SET(CMAKE_INSTALL_PREFIX "/opt/${PACKAGE_NAME}-${PACKAGE_VERSION}"
CACHE PATH "Install path prefix")

That style works for us to provide a default option that can be changed by
the user.  I assume that style will also work to override a variable that has
already been cached, but I don't know for sure.

Alan

=======================================

CMAKE_INSTALL_PREFIX: cmake ../kdelibs -DCMAKE_INSTALL_PREFIX=/opt/kde4 is the equivalent to ./configure --prefix=/opt/kde4