Update mkmf to use perl in environment rather than /usr/bin/perl

[ceblanton]

Currently, mkmf uses the /usr/bin/perl as specified in the shebang: #!/usr/bin/perl

mkmf uses the compiled Config module that fails with an error if the running Perl version doesn't match the Perl used to compile the library.

The solution is to use the Perl in the environment, which should match the Perl used the compile Config.pm: #!/usr/bin/env perl

[underwoo]

@ceblanton will you do the work, or will someone else?

[ceblanton]

Ready in #36