pedantically check the presense of required header files
workaround to successfully detect presense of attr/xattr.h
Merge pull request #236 from reneme/detect_libattr

Check the Presence of libattr1-dev in CMakeLists.txt

This adds (pedantic) checking for the include file attr/xattr.h and thus for the presence of libattr1-dev. Also all other required headers are now checked pedantically.