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
Process mkmf-related stuff only when pilot-link is needed.
Merge pull request #11 from ykasap/fix_extconf

Process mkmf-related stuff only when pilot-link is needed.
Do not assume that EOL is CRLF.
Merge pull request #12 from ikazuhiro/work

Do not assume that EOL is CRLF.
Convert obsolete old-style backquotes.
Merge pull request #13 from ykasap/warning

Convert obsolete old-style backquotes.