Restructure codebase and update build configuration

[marcransome]

• Switch from getopt to libpopt to facilitate unit testing (the cmocka test framework executes all tests for a given test target in the same process and getopt's use of global state makes parsing multiple sets or arguments problematic in this scenario—even when using optreset as described in getopt(3))
• Refactor functions that operate on opaque FlogConfig instances to simplify testing:
  • exit is no longer called internally and it is now the responsibility of the caller to check for error conditions and handle them appropriately (as in main.c)
  • flog_config_new returns NULL and will set errno to an appropriate value for any of several error conditions (see defs.h)
  • assert is conditionally redefined when testing is enabled in order to test for expected assertion failures in tests
• Bump version number to 1.4.0 pending next release milestone