commit 7b1ee33f339bd0d69a7295facda7b3d2b4b55d1a
Author: Sergey V. Lobanov <sergey@lobanov.in>
Date:   Sat Jan 8 14:22:21 2022 +0300

    allow override OS to support cross-compile compilation if build OS
    host OS are different (e.g. build on MacOS for Linux)

--- a/Makefile
+++ b/Makefile
@@ -28,7 +28,7 @@ INSTALLDIR=install
 #
 # Determine OS
 #
-OS:=$(shell uname -s | cut -c -7)
+OS?=$(shell uname -s | cut -c -7)
 
 #
 # Windows rules