[CMake] Comments and improvement on find modules

[Ghislain Vaillant]

Dear CMake users,

I have recently written a set of find modules [1][2][3] for dependencies 
I use for the project I am currently working on. I made sure to write 
them in a style that is close to what is currently maintained in CMake, 
with the goal to submit them for inclusion upstream and benefit the 
whole community.

I wanted to ask you guys for comments regarding their content. Some 
things may be superfluous, others may be missing or improved. Any piece 
of feedback is warmly welcome.

I was also wondering whether there is some sort of best practice with 
regards to the handling of user-defined prefixes for custom locations of 
dependencies. I saw some modules using ${Pkg}_ROOT, others using 
${Pkg}_DIR, some using a cached variable...Could you guys point me to 
what is considered good examples here.

[1] 
https://github.com/ghisvail/IRTK/blob/enh/cmake-find-modules/CMake/FindFLANN.cmake
[2] 
https://github.com/ghisvail/IRTK/blob/enh/cmake-find-modules/CMake/FindLibLBFGS.cmake
[3] 
https://github.com/ghisvail/IRTK/blob/enh/cmake-find-modules/CMake/FindNiftiCLib.cmake

Many thanks,
Ghislain