[CMake] find_package and CMAKE_INSTALL_PREFIX

[Maik Beckmann]

  This is not a question but a finding during a debugging session I like 
to share and a documentation enhancement request.

We have a number of projects that reside in different repositories 
including library projects that are chained up by find_package in 
conjunction with -config.cmake files that each library projects 
installs.  So far we just had this rule:
   The N project builds are independent from each other. All they share 
is a common CMAKE_INSTALL_PREFIX

This strategy has proven to reliable and even noob proof.

Now we started installing our packages on the system and keep working on 
the development version independent from these installation.  Which 
caused some trouble...

If Proj-libFoo is installed to /usr, Proj-libBar depends on it and is 
configured with
   -DCMAKE_INSTALL_PREFIX=$HOME/projects/install
it will pick the one in /usr.
It took me some time to figure where CMAKE_INSTALL_PREFIX is injected 
into the list of prefixes to consider by find_package: 
CMAKE_SYSTEM_PREFIX_PATH in cmFindPackageCommand.cxx
    this->AddCMakePath("CMAKE_SYSTEM_PREFIX_PATH")
CMAKE_SYSTEM_PREFIX_PATH holds a list of prefixes (/usr/local, /usr,...) 
where CMAKE_INSTALL_PREFIX  is the last entry.

That's the reason why our "just point the project to the others by using 
CMAKE_INSTALL_PREFIX" rule fell apart.

The fix is easy: just set CMAKE_PREFIX_PATH to the same location that 
CMAKE_INSTALL_PRFIX is set to.


Guys, please add the role that CMAKE_INSTALL_PREFIX plays for 
find_package to the docs.


Thanks,
Maik