Add script that checks if every biTStream header can be used standalone.
Add script that checks if every biTStream header can be used standalone.

Add headers_test.sh. A script that checks if every biTStream header
can be used standalone (includes all needed headers to implement
its functionality).

kgmoore/bitstream,avble/bitstream,svogl/bitstream,kierank/bitstream,svogl/bitstream,hiroshiyui/bitstream,avble/bitstream,hiroshiyui/bitstream,svogl/bitstream,kgmoore/bitstream,theambient/bitstream_c,kgmoore/bitstream,hiroshiyui/bitstream,theambient/bitstream_c,gfto/bitstream,avble/bitstream,kierank/bitstream,gfto/bitstream