Check RUBY_VERSION to use force_encoding
configure.rb: Don't give up when pilot-link is not installed.
ruby-ext:extconf.rb.in: Require 'mkmf' only when create_makefile is needed.
Merge pull request #9 from ykasap/fix_configure

Fix configure scripts to properly ignore absence of pilot-link library
Merge pull request #8 from tats/bug/ruby18

Check RUBY_VERSION to use force_encoding
Require mkmf to handle $CFLAGS and $LDFLAGS
Merge pull request #10 from tats/bug/mkmf

Require mkmf to handle $CFLAGS and $LDFLAGS