Add hook to create ccov target each time that add_ex.. and add_lib...…

[rlalik]

… is called.

The main motivation for this change was laziness. If I need coverage for each and every target in my project I do not want to double every add_* function with extra to add coverage, I want cmake to do it for me.

[rlalik]

Ok, I need to work on this a bit more. For sure my two other pull-request are also included in this PR (I later split it into separate) here so I would ask to accept these two first:

• one for PLAIN option #36
• second for change of the ccov targets dependencies #37

before proceeding with this. Then I make this PR smaller.