[Cpp] CMake: use variables for specifying install paths consistently

[jmairboeck]

This allows overriding them if needed when the system uses different paths than usual (e.g. Haiku, which uses "develop/headers" for includes instead of "include").

Signed-off-by: Joachim Mairböck [email protected]

[jmairboeck]

I rebased this on top of the changes from #4661.

[hzeller]

@parrt this is sitting here for a while, should it be merged ?

[parrt]

I closed my eyes and hit merge :)