--gc=inf: fix t/op/gc.t and mention gms in Configure --help

gc_type = INF should skip the first gc tests
add local .perldb helper to .gitignore/MANIFEST.SKIP

so that git clean -dxf will not wipe it.
Improve pmc2c deps for PMC/$class.pm libs

depends only on the really used deps.
also sort the deps to generate comparable Makefile