Rephrase some not-supported error messages in pg_hba.conf processing.

In a couple of places we said "not supported on this platform" for cases
that aren't really platform-specific, but could depend on configuration
options such as --with-openssl.  Use "not supported by this build" instead,
as that doesn't convey the impression that you can't fix it without moving
to another OS; that's also more consistent with the wording used for an
identical error case in guc.c.

No back-patch, as the clarity gain is small enough to not be worth
burdening translators with back-branch changes.

src/backend/libpq/hba.c

@@ -850,7 +850,7 @@ parse_hba_line(List *line, int line_num, HbaLine *parsedline)
 #else
 			ereport(LOG,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
-					 errmsg("hostssl not supported on this platform"),
+					 errmsg("hostssl is not supported by this build"),
 			  errhint("Compile with --with-openssl to use SSL connections."),
 					 errcontext("line %d of configuration file \"%s\"",
 								line_num, HbaFileName)));
@@ -1148,7 +1148,7 @@ parse_hba_line(List *line, int line_num, HbaLine *parsedline)
 	{
 		ereport(LOG,
 				(errcode(ERRCODE_CONFIG_FILE_ERROR),
-				 errmsg("invalid authentication method \"%s\": not supported on this platform",
+				 errmsg("invalid authentication method \"%s\": not supported by this build",
 						token),
 				 errcontext("line %d of configuration file \"%s\"",
 							line_num, HbaFileName)));