build: use the standard AM_V_GEN definition for output.

This actually allows for the command to be printed if using `make V=1`

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/src/Makefile.am b/src/Makefile.am
index b79c8b7..36f95ee 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -39,5 +39,4 @@
 endif
 
 crc%gen.c: crcXXgen.c.tpl
-	@echo "  SED    $< -> $@"
-	@sed -e's/XX/$*/g' $< > $@
+	$(AM_V_GEN)sed -e's/XX/$*/g' $< > $@