Make driver::driver::parse_cfgspecs parse any variety of meta_item

All current meta items types (word, name-value, list) are now
properly parsed by rustc --cfg command line. Fixes #2399

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #5433 : lucab/rust/lucab/cfgspec, r=graydon

All current meta items types (word, name-value, list) are now
properly parsed by rustc --cfg command line. Fixes #2399