Merge pull request #10 from ixjlyons/fix-mandir

Honor DESTDIR in man page installation.

Makefile

 PREFIX=$(DESTDIR)/usr
 BINDIR=$(PREFIX)/bin
-MANDIR=/usr/share/man/man1
+MANDIR=$(PREFIX)/share/man/man1
 
 CC=gcc
 CFLAGS=-std=c89 -O2 -pedantic -Wall -I"./include"