build: Delete generated parse-datetime.c file, use AM_V_GEN

Bison is a well known external dependency, so just require it.
Including the generated content in git means it may or may not
be regenerated based randomly on timestamps, etc.

Also use `$(AM_V_GEN)` so we get prettier output.

Makefile-ostree.am

@@ -85,8 +85,8 @@ ostree_SOURCES += \
         src/ostree/ot-remote-builtin-refs.c \
 	$(NULL)
 
-src/ostree/parse-datetime.c: src/ostree/parse-datetime.y
-	$(YACC) $< -o $@
+src/ostree/parse-datetime.c: src/ostree/parse-datetime.y Makefile
+	$(AM_V_GEN) $(YACC) $< -o $@
 
 
 ostree_bin_shared_cflags = $(AM_CFLAGS) -I$(srcdir)/src/libotutil -I$(srcdir)/src/libostree -I$(srcdir)/src/ostree \