configure: dup frozen strings from ARGV

Strings being brought in from ARGV are frozen; however, the configure
script expects to be able to modify some of the strings being passed in.
This leads to "can't modify frozen String" exceptions when attempting
to pass certain options, such as --libdir. Duping the strings returns
non-frozen copies.
Merge pull request #2658 from mistydemeo/configure_argv_frozen

Configure script attempts to modify frozen strings from ARGV