Keep workaround for no-dist-built-files not being available

As no-dist-built-files was only introduced in Automake 1.16.4,
we cannot rely on that feature being present and need to continue
using our own workaround.

This hooks the dist-hook in a more robust fashion in the face
of multiple dist-hook targets (and versioninfo.mk already uses
one).

src/Makefile.am

@@ -31,15 +31,19 @@ EXTRA_DIST   = \
 	avrdude.spec \
 	bootstrap
 
-CLEANFILES = \
-	config_gram.c \
-	config_gram.h \
-	lexer.c
+BUILT_SOURCES  =
+CLEANFILES     =
+
+built_sources  =
+built_sources += config_gram.c
+built_sources += config_gram.h
+built_sources += lexer.c
+
+BUILT_SOURCES += $(built_sources)
+CLEANFILES    += $(built_sources)
 
 include build-helpers/versioninfo.mk
 
-BUILT_SOURCES = $(CLEANFILES)
-
 #SUBDIRS      = doc
 #DIST_SUBDIRS = doc
 
@@ -77,11 +81,12 @@ lib_LTLIBRARIES = libavrdude.la
 # https://savannah.nongnu.org/bugs/index.php?func=detailitem&item_id=15536
 #
 # for why we don't want to have them.
-dist-hook:
-	rm -f \
-	$(distdir)/lexer.c \
-	$(distdir)/config_gram.c \
-	$(distdir)/config_gram.h
+#
+# We could avoid this dist-hook altogether if we could require
+# Automake >= 1.16.4 and just use its no-dist-built-sources flag.
+dist-hook: dist-hook-no-dist-built-sources-workaround
+dist-hook-no-dist-built-sources-workaround:
+	cd "$(distdir)" && rm -f $(built_sources)
 
 libavrdude_a_SOURCES = \
 	config_gram.y \