Auto merge of #2913 - cuviper:disable-option-checking, r=alexcrichton

configure: Support --disable-option-checking

This is mirroring rust-lang/rust#31169.  The RPM %configure macro sets a
lot of useful paths for typical configure scripts, but some of these
values are not recognized here in Cargo.  It's nice to have an option to
ignore those, rather than failing on `validate_opt`.