CHECK include directories only added in case it has been found

[rpoisel]

Executing include_directories() with CHECK_INCLUDE_DIRS being set to NOTFOUND leads to an error. Therefore, an additional condition has been introduced.

[rpoisel]

Furthermore, I'd like to encourage you to prefer using target_include_directories() over include_directories() - see https://cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#transitive-usage-requirements for details. Or here: https://rix0r.nl/blog/2015/08/13/cmake-guide/

[paroga]

Added via bca88b1.
Ad target_include_directories(): I made the change already local, but it requires at least CMake 2.8.11 but Ubuntu 12.04 ships with 2.8.7. :-(

[coveralls]

Coverage remained the same at 97.565% when pulling 3b016f8 on rpoisel:check into 6f071e6 on WebProcessControl:master.

[rpoisel]

Would it be possible to merge this pull request? It does not use target_include_directories() and I cannot cross compile your library due to unsatisfied dependencies now. :-(

[paroga]

Did you see commit bca88b1, which is in master already?

[rpoisel]

Now I saw it. Sorry and thx! :-)