Working around CMAKE brain-damage

[Allan Sandfeld Jensen]

Hi

I've been trying to resolve a problem I got when configuring varies KDE 4 
modules. CMake complains that it can not sane search paths for some libraries 
in particular qimageblitz and polkitqt. The reason it can set the search path 
is quite clear, CMake has for some reason found the system version of the 
libraries in stead of the development versions. When rpath is set with 
development dirs before system-dirs, linking will choose the correct 
development version and not the wrong system version CMake has picked.

I've been debugging the modules and think I've found the source of the 
problem. The problem is a combination of that we do and what cmake does.

We do:
1. Find the module using pkgconfig
2. Supply the pkg-config paths to cmake functions find_path and find_library

What does wrong is that, find_path and find_library search the default system 
paths BEFORE searching the paths that you have asked it to search in. The also 
creates serious issues as we use pkg-config to ensure we have picked the right 
version of a library but since the cmake find_ functions use their own broken 
logic, they can easily pick includes and lib-paths that differ from the pkg-
config result we have found and verified.

There is a number of solutions to fix the problem:
1. Never use find_path and find_library after pkg-config. We have found the 
module using pkg-config so trust it to return correct paths.
2. Run find_path and find_library with the NO_DEFAULT_PATH flag. This will 
make them not search the default paths and only the specified paths. To still 
have proper fail-over we could call them again afterwards to search the 
defaults incase a path haven't been found yet.
3. Instruct developers to set always set the new enviroment variables $INCLUDE 
and $LIB when setting $PKG_CONFIG_PATH.

Attached is an example of solution number 2.

Greetings
`Allan

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