Preliminary changes for UTF-8 support.

This patch includes everything from the patch by mcmtroffaes
in #42 except the compUTF8 option itself, which is commented
out pending release of a version of pcre-regex-builtin that supports
it.

When a supporting version is released, we can remove the comment
here, conditionally on the version of pcre-regex-builtin.

See #42.