As indicated in #300 Debian don't allow linking dynamically with libbfd because it is not considered stable enough.

This means that #255 need to be reverted.

• Revert changes by Fix #251: cgreen-runner handle non-lib files #255 and related changes
• Add run-time check for file type (to avoid Giving something not a library to cgreen-runner will result in error from nm #251
• Add build-time check for existence of nm (instead of libbfd)
• Release 1.6.0