fix flex.1 dependency

git clean -fdx && ./autogen.sh && \
  mkdir -p ../build && cd ../build && ../flex/configure && make -j10 && \
  make dist

failed with:

>  help2man: can't get `--help' info from ../../flex/src/flex

Note:
  There is no dependency except for the flex binary.

doc/Makefile.am

@@ -22,9 +22,6 @@ CLEANFILES = \
 	flex.vr \
 	flex.vrs
 
-$(dist_man_MANS): $(top_srcdir)/configure.ac $(top_srcdir)/src/flex.skl $(top_srcdir)/src/options.c $(top_srcdir)/src/options.h
-	for i in $(dist_man_MANS) ; do \
-	$(help2man) --name='$(PACKAGE_NAME)' \
-	--section=`echo $$i | sed -e 's/.*\.\([^.]*\)$$/\1/'` \
-	 $(top_srcdir)/src/flex$(EXEEXT) > $$i || rm -f $$i ; \
-	done
+flex.1 : ../src/flex$(EXEEXT)
+	$(help2man) --name='$(PACKAGE_NAME)' --section=1 --output=$@ $< || \
+		rm -f $@