Makefile: detect running OS

Makefile

@@ -2,6 +2,7 @@
 # slstatus - suckless status monitor
 .POSIX:
 
+include os.mk
 include config.mk
 
 REQ = util

README

@@ -36,8 +36,9 @@ In order to build slstatus you need the Xlib header files.
 
 Installation
 ------------
-Edit config.mk to match your local setup (slstatus is installed into the
-/usr/local namespace by default). Uncomment OSSLIBS on OpenBSD.
+Run ./configure first to detect your operating system.
+Optionally edit config.mk to match your local setup (slstatus is installed into
+the /usr/local namespace by default). Uncomment OSSLIBS on OpenBSD.
 
 Afterwards enter the following command to build and install slstatus (if
 necessary as root):

configure

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+printf 'OS = %s\n' "$(uname)" > os.mk