1. In "display_test_output": when printing dependencies without
linkage, use "Dependencies with no linkage" string instead of "Possible
unnecessary dependencies" for consistency with "display_normal_output"
2. In "check_dylibs": keep track of and skip repetitive checking of
those dylibs that have been previously checked. This is applicable when
different executables/libraries are linked against the same libraries.
3. In "check_undeclared_deps": when there are missing libraries,
corresponding dependencies should be excluded from the list of
dependencies with no linkage.
