jscott/which: A small implementation of 'which', a utility showing where programs can be found

A small implementation of 'which', a utility showing where programs can be found

John Scott 61fc96e376 Define _XOPEN_SOURCE to 700.		3 lat temu
meson.build	d00a50c71b Stop assuming strerror() is thread-safe.	3 lat temu
which.c	61fc96e376 Define _XOPEN_SOURCE to 700.	3 lat temu