cmake and build script fixes
- make the build process callable without the build_pcf script by deducing the value of OPENAIRCN_DIR
- use the CMake directive for cpp17 rather than the gcc-specific option
- fix the error 'can not remove $3, is a directory'
- include errors and warnings in the log file when verbose
- do not repeat build flags by adding CMAKE_FLAGS to CMAKE_FLAGS_${BUILD_TYPE}
- git ignore the build files and directories
Edited by Romain Lacroix