For the VPATH builds done in Userland, tmux fails to find an awk script used
to prepare the man page.  This patch was submitted to the maintainer on 
2014-09-16, he said he has already fixed this problem in the development version.

--- ORIGINAL/Makefile.in	2014-02-20 15:11:49.000000000 -0800
+++ tmux-1.9/Makefile.in	2014-11-20 11:28:36.648421671 -0800
@@ -1221,7 +1221,7 @@
 			>$(srcdir)/tmux.1.mdoc; \
 	else \
 		sed -e "s|@SYSCONFDIR@|$(sysconfdir)|g" $(srcdir)/tmux.1| \
-			$(AWK) -fmdoc2man.awk >$(srcdir)/tmux.1.man; \
+			$(AWK) -f$(srcdir)/mdoc2man.awk >$(srcdir)/tmux.1.man; \
 	fi
 	$(MKDIR_P) $(DESTDIR)$(mandir)/man1
 	$(INSTALL_DATA) $(srcdir)/tmux.1.@MANFORMAT@ \