src: Add threadlib library for parallel job execution

src: Add threadlib library for parallel job execution

Add new internal static library libthread.a that provides infrastructure
for eu-* tools to run functions concurrently using pthreads.

threadlib.c manages per-job threads as well as per-job buffers for stdout
output.  Output for each job is printed to stdout in the order that the
jobs were added to the job queue.  This helps preserve the order of
output when parallelization is added to an eu-* tool.

threadlib.h declares functions add_job and run_jobs. Jobs are added to
a threadlib.c internal job queue using add_job. run_jobs concurrently
executes jobs in parallel.

eu-readelf now links against libthread.a when elfutils is configured
with --enable-thread-safety.

	* src/Makefile.am: libthread.a is compiled and and linked with
	readelf when USE_LOCKS is defined.
	* src/threadlib.c: New file. Manages job creation, concurrent
	execution and output handling.
	* src/threadlib.h: New file. Declares functions add_job and
	run_jobs.

Signed-off-by: Aaron Merey <amerey@redhat.com>