It was missing a few files.
Should also fix CI runs.

* Since case-insensitivity is the default, you couldn't effectively
  search for non-ANSI characters, although character classes were not affected.
* This was a terex bug, i.e. wasn't broken in v2.5.2.
* Test cases have been added.
  The test case for searching for ^E (5) has been simplified using `^E<5>`.

* terex disables assertions by default unless you add `-DREG_DEBUG`.
  Since we heavily modified the original by Henry Spencer it makes sense
  to enable assertions.
* dlmalloc will still be built without assertions even if --enable-debug
  is given since that has a significant speed impact and I consider
  dlmalloc to be rock solid. It would need `-DDEBUG=1` to enable assertions
  (among other things).
  We only disable additional checks in dlmalloc if --disable-debug.

dlmalloc had one bogus unused variable warning, so we also added `-Wno-unused-but-set-variable`.
We don't want to change upstream sources unless absolutely necessary.

* The previous checks for interruptions only helped in a few corner cases
  like for very high search-repeat counts or during backwards searches across
  the entire buffer.
* But even with terex' more predictable runtime properties
  a single regex execution can hang quite a long time.
  E.g. `S^EM^X$` on a huge buffer or even more so with backreferences as in
  `S^~(.*)\1$`.
* We now use the new tere_set_is_interrupted_cb() to register
  teco_interface_is_interrupted(). Types should be compatible as long
  as gboolean resolves to int.
* It's no longer necessary to manually check for teco_interface_is_interrupted()
  since tere_exec() now returns REG_EINTR in case the callback returned TRUE
  in which case it's handled by teco_error_regex_set().

* terex is based on Henry Spencer's regular expression engine for Tcl.
  It is a hybrid NFA/DFA design which has better worst-time runtimes than
  the backtracking PCRE. Memory usage is also limited and can no longer
  increase catastrophically.
* It should no longer be possible to crash SciTECO with pathological
  searches.
* Since it reliably supports partial matches (REG_EXPECT) we can
  now enable the new backwards-search algorithm by default.
  This used to be broken because of a glib bug, which I already
  fixed. It would however take a long time until this ends up
  on the majority of glib installations.
* Regexp executions can still be quite slow if you are looking
  for a pattern at the end of a huge file, which can hang the editor,
  but this can now at least theoretically be solved by adding
  hooks into terex to poll for interruptions.
* We can now also get rid of a TECO-pattern to regexp translation
  step by directly generating terex tokens (TODO).
* Performance-wise terex appears to be slower than PCRE for simple
  forward searches even when linking everything with optimzations (FIXME).
* Having a stand-alone regular expression engine is also a huge
  step in getting rid of glib.

See also: https://git.fmsbw.de/terex/about/