CMakeProject: Fix CXX Flags parsing for ninja projects

For ninja projects the "build.ninja" file is parsed to extract the cxx
flags for each target. This file is located in the working directory
of the "all" target, however since commit "65c113bc" qtcreator searches
in the build directory of the current target. I have restored the
search behavior to the previous state and added some code to ensure
that the parsed flags really belong to the target

Change-Id: I7cc7f6dbd4f12aec698133206da889037131bb13
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>