Builds two convenience libraries: libterex.la and libteurex.la.

This will be the SciTECO regular expression engine and we have to fork
it permanently.

* They are built with `-DREGEX_UTF8` instead of `-DREGEX_WCHAR`.
  Functions are called reg_ucomp() and reg_uexec() instead for consistency.
  The library is now called libhsurex.so instead of libhswrex.so.
* The `chr` type is now always `unsigned char`.
  As a result many other uses of the `chr` type had to be changed to pchr
  (which is always large enough to hold a byte or wide character).
  Generally we try to keep code changes as small as possible since
  we may have to backport changes from the Tcl codebase or contribute
  patches to the Tcl project.