[cmake] Cosmetics: change commands to lowercase

[hudokkow]

Cosmetics only and low priority.
Script used to automate it can be found here. Might be useful for others.

@fetzerch, @wsnipex for review

[fetzerch]

Personally I'm quite in favor of strict rules. One thing that would additionally make sense:
if("${OS}") is simply if(OS).

For the files that are only used in the main CMake build system, I'd say: go for it. (there's not much git history anyway). For the binary addon buildsystem it still has my +1, even though I see a small disadvantage of git blame being a bit more difficult to use.

[fetzerch]

One more thing might be to replace "if( " and " )" with "if(" and ")"?

[hudokkow]

If no one objects, this is going in tomorrow.
jenkins build this with addons please