fix OpenBSD compiler command

[Veraellyunjie]

On OpenBSD 7.1 gcc is installed as egcc

pkg_info -L gcc
...
/usr/local/bin/egcc
...

With original Makefile you get:

gmake: gcc: No such file or directory

Make Makefile aware of it

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Also, original README.md doesn't mention the gcc among tools to be installed