Fix warning that NSEC_PER_SEC is defined on OS X
Compile error on OS X - missing bracket
Improve wording of AC_DEFINEs in configure.ac
Use AS_CASE instead of case/esac
Use tabs instead of whitespaces in configure.ac

Additionally, do not use line breaks in AS_HELP_STRING, AC_MSG_ERROR, or
AC_MSG_WARN macros since m4 cannot handle trailing whitespace properly.
Clock_gettime implementation for OS X

OS X does not have clock_gettime, use clock_get_time instead.
Non-recursive automake

According www.flameeyes.eu/autotools-mythbuster/automake/nonrecursive.html
non-recursive make files have several advantages over the recursive
'classic' version. One advantage is that it exists only a single Makefile.am
to edit, and a single Makefile.in to be processed during ./configure
mrproper make target

Pre-existing make *clean targets are not suitable to delete all generated
files by 'autoreconf -i'. mrproper will clean up all those files.