TRE: Check for 32-bit wchar_t at configure time

This check used to be in src/elinks.h.  Move it to configure.in so
that (1) the result can be logged and (2) ELinks won't even link with
TRE if wchar_t prevents its use.

Also, rename HAVE_TRE_REGEX_H to CONFIG_TRE, to reflect that it is not
always defined if the header exists.

src/elinks.h

@@ -29,30 +29,6 @@
 #define DEBUG_MEMLEAK
 #endif
 
-
-/* When CONFIG_UTF8 is defined, src/viewer/text/search.c makes a string
- * of unicode_val_T and gives it to regwexec(), which expects a string
- * of wchar_t.  If the unicode_val_T and wchar_t types are too different,
- * this won't work, so try to detect that and disable regexp operations
- * entirely in that case.
- *
- * Currently, this code only compares the sizes of the types.  Because
- * unicode_val_T is defined as uint32_t and POSIX says bytes are 8-bit,
- * sizeof(unicode_val_T) is 4 and the following compares SIZEOF_WCHAR_T
- * to that.
- *
- * C99 says the implementation can define __STDC_ISO_10646__ if wchar_t
- * values match ISO 10646 (or Unicode) numbers in all locales.  Do not
- * check that macro here, because it is too restrictive: it should be
- * enough for ELinks if the values match in the locales where ELinks is
- * actually run.  */
-
-#ifdef CONFIG_UTF8
-#if SIZEOF_WCHAR_T != 4
-#undef HAVE_TRE_REGEX_H
-#endif
-#endif
-
 /* This maybe overrides some of the standard high-level functions, to ensure
  * the expected behaviour. These overrides are not system specific. */
 #include "osdep/stub.h"