The travis clang-8 reports a SIGILL with the code:

    std::string str = "    ";
    Ensures(baryonyx::left_trim(str) == "");

Fix the problem using .empty() instead of operator==.

- travis:
  - use `-O2` seems to fix a segfault in the testlib unit test.
  - execute test with gdb

Error seems to unrecognize the ``[[maybe_unused]]` token with the `if
constexpr`:

error: expected unqualified-id before ‘[’ token

     debug_logger([[maybe_unused]] const std::string_view name) noexcept

lib/src/itm-common.hpp:1796:18: error: expected ‘)’ before ‘[’ token

     debug_logger([[maybe_unused]] const std::string_view name) noexcept

                 ~^