Properly use $(AWK) in Makefile, not 'awk'

Fix for commit 498f130756.

Backpatch-through: master

doc/src/sgml/Makefile

@@ -158,7 +158,7 @@ XSLTPROC_FO_FLAGS += --stringparam img.src.path '$(srcdir)/'
 %.pdf: %.fo $(ALL_IMAGES)
 	@# There is no easy way to pipe output and capture its return code, so output a special string on failure.
 	{ LANG=C $(FOP) -fo $< -pdf $@ 2>&1; [ "$$?" -ne 0 ] && echo "FOP_ERROR"; } | \
-	awk 'BEGIN { warn = 0 }  ! /^FOP_ERROR$$/ { print }  /not available in font/ { warn = 1 }  \
+	$(AWK) 'BEGIN { warn = 0 }  ! /^FOP_ERROR$$/ { print }  /not available in font/ { warn = 1 }  \
 	END { if (warn != 0) print("\nFound characters that cannot be output in the PDF document;  see README.non-ASCII"); \
 	if ($$0 ~ /^FOP_ERROR$$/) { exit 1} }' 1>&2