Copy the scan.l file to the build directory so that the .c file can be
successfully generated.

Run help2man on the flex file in the build directory to generate the man page.

Make sure that we always use /usr/gnu/bin/m4 rather than what is available
via the M4 environment variable or what's in the PATH environment variable.

These changes have been sent upstream. See email thread starting at:

  https://sourceforge.net/p/flex/mailman/message/35103749/

--- flex-2.6.1/src/Makefile.am.orig	2016-05-19 06:21:16.631609293 -0700
+++ flex-2.6.1/src/Makefile.am	2016-05-19 14:19:29.394717203 -0700
@@ -87,7 +87,7 @@
 	mv skel.c.tmp $(srcdir)/skel.c
 
 stage1scan.l: scan.l
-	cp $(srcdir)/scan.l $(srcdir)/stage1scan.l
+	cp $(srcdir)/scan.l $(top_builddir)/src/stage1scan.l
 
 stage1scan.c: stage1scan.l stage1flex$(EXEEXT)
 	$(top_builddir)/src/stage1flex$(EXEEXT) -o $@ $<
--- flex-2.6.1/doc/Makefile.am.orig	2016-05-19 06:45:26.670565152 -0700
+++ flex-2.6.1/doc/Makefile.am	2016-05-19 06:45:45.734263762 -0700
@@ -26,5 +26,5 @@
 	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 $$i ; \
+	 $(top_builddir)/src/flex$(EXEEXT) > $$i || rm -f $$i ; \
 	done
--- flex-2.6.1/src/main.c.orig	2016-05-19 14:36:18.569346881 -0700
+++ flex-2.6.1/src/main.c	2016-05-20 07:59:04.169569771 -0700
@@ -348,6 +348,7 @@
 
     /* Setup the filter chain. */
     output_chain = filter_create_int(NULL, filter_tee_header, headerfilename);
+#ifndef __sun
     if ( !(m4 = getenv("M4"))) {
 	    char *slash;
 		m4 = M4;
@@ -389,6 +390,9 @@
 		}
 	}
     filter_create_ext(output_chain, m4, "-P", 0);
+#else
+    filter_create_ext(output_chain, "/usr/gnu/bin/m4", "-P", 0);
+#endif
     filter_create_int(output_chain, filter_fix_linedirs, NULL);
 
     /* For debugging, only run the requested number of filters. */