Be bsdmake friendly : don't embed $() in another $()

src/Makefile.am

@@ -391,7 +391,8 @@ SOURCES_libvlc_control = \
 misc/revision.c:
 	rm -f $@ $@.tmp
 	echo "/* AUTOGENERATED FILE - DO NOT EDIT */" > $@.tmp
-	REVISION="$$((LANG=C git --git-dir=$(top_srcdir)/.git show-ref -s HEAD 2>/dev/null || echo exported) | cut -b -10)"; \
+	REVISION="$$(LANG=C git --git-dir=$(top_srcdir)/.git show-ref -s HEAD 2>/dev/null || echo exported)"; \
+	REVISION="$$(echo $$REVISION|cut -b -10)"; \
 	echo "const char psz_vlc_changeset[] = \"$$REVISION\";" >> $@.tmp
 	mv -f $@.tmp $@