This is a port of NetBSD m4 (4.0) to Linux.
It compiles with a couple warnings using musl.
When built as a shared binary it is ~22k (-Os, stripped binary).

The original code is copyright by the UC Regents and the NetBSD Foundation.
Some modifications have been made.

All changes are hereby released as public domain;
In the event that this is not permitted by law, the following license applies:
This code may be used, modified, and distributed in any form, whether modified
or unmodifed, without restriction or credit, for any purpose whatsover.

Regardless of whether this code is used under license or as public domain, 
all warrantees are explicitly disclaimed, INCLUDING THE IMPLIED WARRANTEES
OF MERCHANTABILITY AND FITNESS FOR ANY PURPOSE.