BackwardConfig.cmake forcing dependency on BACKWARD_LIBRARIES

[thfabian]

When including the BackwardConfig.cmake via find_package:

find_package(backward NO_MODULE REQUIRED)

it fails to properly detect backward if neither libdw nor libbfd are found as in this case BACKWARD_LIBRARIES is empty, hence the failure in find_package_handle_standard_args in https://github.com/bombela/backward-cpp/blob/master/BackwardConfig.cmake#L121 . Given that both libraries are optional, the failure should not be a hard error (even with REQUIRED)

[BotellaA]

Any progress on merging this commit into the current repo?