pg_trgm's set_limit() now uses SetConfigOption()

Deprecated set_limit() is modified to use SetConfigOption() to set
similarity_threshold which is actually an instance of
pg_trgm.similarity_threshold GUC variable. Previous coding directly sets
similarity_threshold what could cause an inconsistency between states of
actual variable and GUC representation.

Per gripe from Tom Lane

contrib/pg_trgm/trgm_op.c

@@ -9,6 +9,7 @@
 
 #include "catalog/pg_type.h"
 #include "tsearch/ts_locale.h"
+#include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/pg_crc.h"
 
@@ -80,12 +81,17 @@ Datum
 set_limit(PG_FUNCTION_ARGS)
 {
 	float4		nlimit = PG_GETARG_FLOAT4(0);
+	char	   *nlimit_str;
+	Oid			func_out_oid;
+	bool		is_varlena;
+
+	getTypeOutputInfo(FLOAT4OID, &func_out_oid, &is_varlena);
+
+	nlimit_str = OidOutputFunctionCall(func_out_oid, Float4GetDatum(nlimit));
+
+	SetConfigOption("pg_trgm.similarity_threshold", nlimit_str,
+					PGC_USERSET, PGC_S_SESSION);
 
-	if (nlimit < 0 || nlimit > 1.0)
-		ereport(ERROR,
-				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
-				 errmsg("wrong threshold, should be between 0 and 1")));
-	similarity_threshold = nlimit;
 	PG_RETURN_FLOAT4(similarity_threshold);
 }