Utils::FileName: Fix append("") appending a '/'

Change-Id: I2767a901220711dbba6b34c36e256d282f468778
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
Cmake: Fix include paths for some cmake projects

The affected cmake projects have the CMakeLists.txt in a subdirectory
of the project's root. We can't figure out the right project part in
that case. Falling back to the "all" target is wrong though, since
that has not every include path. Instead search for a good target
that has more include paths.

Change-Id: I1a874042fcb9533888a41c001fbf8adc2aa90a39
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>