multiple changes, expand extended description

- ignore kextlibs errors
- check for tool existence in autogen.sh script
- clean up after running autogen.sh script
- check filesystem-stuff in configure.ac
- check architecture-related (e.g. Apple universal builds) stuff in
configure.ac
- add commentary in configure.ac
- add IOKit checks in configure.ac
- check accepted compiler flags in configure.ac
- create a "target.h" header
- display output variables at end of configure script