Install manual page to the correct directory.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -25,7 +25,7 @@ RM ?= rm -f
 export BUILDDIR ?= .
 DESTDIR ?=
 PREFIX ?= /usr
-MANDIR ?= $(PREFIX)/share/man
+MANDIR ?= $(PREFIX)/man
 
 BIN := $(BUILDDIR)/bin
 OBJ := $(BUILDDIR)/obj