Assume wcstombs(), towlower(), and sibling functions are always present.

These functions are required by SUS v2, which is our minimum baseline
for Unix platforms, and are present on all interesting Windows versions
as well.  Even our oldest buildfarm members have them.  Thus, we were not
testing the "!USE_WIDE_UPPER_LOWER" code paths, which explains why the bug
fixed in commit e6023ee7f escaped detection.  Per discussion, there seems
to be no more real-world value in maintaining this option.  Hence, remove
the configure-time tests for wcstombs() and towlower(), remove the
USE_WIDE_UPPER_LOWER symbol, and remove all the !USE_WIDE_UPPER_LOWER code.
There's not actually all that much of the latter, but simplifying the #if
nests is a win in itself.

Discussion: https://postgr.es/m/20170921052928.GA188913@rfd.leadboat.com

src/backend/tsearch/regis.c

@@ -178,7 +178,6 @@ RS_free(Regis *r)
 	r->node = NULL;
 }
 
-#ifdef USE_WIDE_UPPER_LOWER
 static bool
 mb_strchr(char *str, char *c)
 {
@@ -209,10 +208,6 @@ mb_strchr(char *str, char *c)
 
 	return res;
 }
-#else
-#define mb_strchr(s,c)	( (strchr((s),*(c)) == NULL) ? false : true )
-#endif
-
 
 bool
 RS_execute(Regis *r, char *str)

src/backend/tsearch/ts_locale.c

@@ -21,8 +21,6 @@
 static void tsearch_readline_callback(void *arg);
 
 
-#ifdef USE_WIDE_UPPER_LOWER
-
 int
 t_isdigit(const char *ptr)
 {
@@ -86,7 +84,6 @@ t_isprint(const char *ptr)
 
 	return iswprint((wint_t) character[0]);
 }
-#endif							/* USE_WIDE_UPPER_LOWER */
 
 
 /*
@@ -244,17 +241,12 @@ char *
 lowerstr_with_len(const char *str, int len)
 {
 	char	   *out;
-
-#ifdef USE_WIDE_UPPER_LOWER
 	Oid			collation = DEFAULT_COLLATION_OID;	/* TODO */
 	pg_locale_t mylocale = 0;	/* TODO */
-#endif
 
 	if (len == 0)
 		return pstrdup("");
 
-#ifdef USE_WIDE_UPPER_LOWER
-
 	/*
 	 * Use wide char code only when max encoding length > 1 and ctype != C.
 	 * Some operating systems fail with multi-byte encodings and a C locale.
@@ -300,7 +292,6 @@ lowerstr_with_len(const char *str, int len)
 		Assert(wlen < len);
 	}
 	else
-#endif							/* USE_WIDE_UPPER_LOWER */
 	{
 		const char *ptr = str;
 		char	   *outptr;

src/backend/tsearch/wparser_def.c

@@ -241,11 +241,9 @@ typedef struct TParser
 	/* string and position information */
 	char	   *str;			/* multibyte string */
 	int			lenstr;			/* length of mbstring */
-#ifdef USE_WIDE_UPPER_LOWER
 	wchar_t    *wstr;			/* wide character string */
 	pg_wchar   *pgwstr;			/* wide character string for C-locale */
 	bool		usewide;
-#endif
 
 	/* State of parse */
 	int			charmaxlen;
@@ -294,8 +292,6 @@ TParserInit(char *str, int len)
 	prs->str = str;
 	prs->lenstr = len;
 
-#ifdef USE_WIDE_UPPER_LOWER
-
 	/*
 	 * Use wide char code only when max encoding length > 1.
 	 */
@@ -323,7 +319,6 @@ TParserInit(char *str, int len)
 	}
 	else
 		prs->usewide = false;
-#endif
 
 	prs->state = newTParserPosition(NULL);
 	prs->state->state = TPS_Base;
@@ -360,15 +355,12 @@ TParserCopyInit(const TParser *orig)
 	prs->charmaxlen = orig->charmaxlen;
 	prs->str = orig->str + orig->state->posbyte;
 	prs->lenstr = orig->lenstr - orig->state->posbyte;
-
-#ifdef USE_WIDE_UPPER_LOWER
 	prs->usewide = orig->usewide;
 
 	if (orig->pgwstr)
 		prs->pgwstr = orig->pgwstr + orig->state->poschar;
 	if (orig->wstr)
 		prs->wstr = orig->wstr + orig->state->poschar;
-#endif
 
 	prs->state = newTParserPosition(NULL);
 	prs->state->state = TPS_Base;
@@ -393,12 +385,10 @@ TParserClose(TParser *prs)
 		prs->state = ptr;
 	}
 
-#ifdef USE_WIDE_UPPER_LOWER
 	if (prs->wstr)
 		pfree(prs->wstr);
 	if (prs->pgwstr)
 		pfree(prs->pgwstr);
-#endif
 
 #ifdef WPARSER_TRACE
 	fprintf(stderr, "closing parser\n");
@@ -437,8 +427,6 @@ TParserCopyClose(TParser *prs)
  *	- if locale is C then we use pgwstr instead of wstr.
  */
 
-#ifdef USE_WIDE_UPPER_LOWER
-
 #define p_iswhat(type)														\
 static int																	\
 p_is##type(TParser *prs) {													\
@@ -536,31 +524,6 @@ p_iseq(TParser *prs, char c)
 	Assert(prs->state);
 	return ((prs->state->charlen == 1 && *(prs->str + prs->state->posbyte) == c)) ? 1 : 0;
 }
-#else							/* USE_WIDE_UPPER_LOWER */
-
-#define p_iswhat(type)														\
-static int																	\
-p_is##type(TParser *prs) {													\
-	Assert( prs->state );													\
-	return is##type( (unsigned char)*( prs->str + prs->state->posbyte ) );	\
-}	\
-																			\
-static int																	\
-p_isnot##type(TParser *prs) {												\
-	return !p_is##type(prs);												\
-}
-
-
-static int
-p_iseq(TParser *prs, char c)
-{
-	Assert(prs->state);
-	return (*(prs->str + prs->state->posbyte) == c) ? 1 : 0;
-}
-
-p_iswhat(alnum)
-p_iswhat(alpha)
-#endif							/* USE_WIDE_UPPER_LOWER */
 
 p_iswhat(digit)
 p_iswhat(lower)
@@ -785,8 +748,6 @@ p_isspecial(TParser *prs)
 	if (pg_dsplen(prs->str + prs->state->posbyte) == 0)
 		return 1;
 
-#ifdef USE_WIDE_UPPER_LOWER
-
 	/*
 	 * Unicode Characters in the 'Mark, Spacing Combining' Category That
 	 * characters are not alpha although they are not breakers of word too.
@@ -1050,7 +1011,6 @@ p_isspecial(TParser *prs)
 				StopHigh = StopMiddle;
 		}
 	}
-#endif
 
 	return 0;
 }