The --version-sort option requires strverscmp(), but this is a glibc
extension that does not exist on OpenBSD and other systems. To enable
--version-sort on those systems, provide an internal implementation of
strverscmp(). The implementation is from the musl C library and is
MIT-licensed.

The build process remains the same: the verscmp flag should be set to 1
only if strverscmp() is available in libc. If verscmp is 0, then the
internal implementation is used.

This allows finer control of which files are compiled and additionally
will silence some -Wpedantic warnings when "empty" files are compiled.

No need to edit config.mk to set addictional CFLAGS/LDFLAGS or change the DESTDIR etc. now

Use config.mk for configuration, both GNU make and FreeBSD make are supported.