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.
I didn't read the code of the tre library, but I suppose that when sizes of
wchar_t and unicode_val_T are equal it will work fine.
[ From bug 1060 attachment 508. --KON ]
