[Cin] arm/termux series (possibly broken on x86)

[mnieuw at zap.a2000.nl]

If you are modifying configure.ac anyway, I suggest to comment out the
line about "no-unknown-warning-option" ?
It causes no end of useless warnings.

Also, the detection of the number of cpus (about line 143) is done by a
very specific Linux method /proc/cpuinfo) , which does not work on
FreeBSD and macOS.

It would probably be better to replace it with "nproc", which can be
made available on FreeBSD via an alias, and on macOS after installing
some stuff with the "HomeBrew" package manager.

MatN