PDF docs build: avoid spurious "warn" in build logs.

Improve on e4c8865 so that the string "warn" appears in
the output when there's a problem, and not when there isn't.
This should silence noise I've been seeing in my buildfarm
warning scraper.

Author: tglsfdc

doc/src/sgml/Makefile

@@ -158,9 +158,9 @@ XSLTPROC_FO_FLAGS += --stringparam img.src.path '$(srcdir)/'
 # There is no easy way to pipe output and capture its return code,
 # so add a final line to report the return code.
 %.pdf: %.fo $(ALL_IMAGES)
-	{ LANG=C $(FOP) -fo $< -pdf $@ 2>&1; echo "FOP_RETURN $$?"; } | \
+	@ { LANG=C $(FOP) -fo $< -pdf $@ 2>&1; echo "FOP_RETURN $$?"; } | \
 	$(AWK) 'BEGIN { warn = 0 }  $$1 != "FOP_RETURN" { print }  /not available in font/ { warn = 1 }  \
-	END { if (warn != 0) print("\nFound characters that cannot be output in the PDF document;  see README.non-ASCII"); \
+	END { if (warn != 0) print("\nWarning: found characters that cannot be output in the PDF document;  see README.non-ASCII"); \
 	if ($$2 != 0) { exit $$2} }' 1>&2