sort: consistently diagnose access issues to --compress-program

sort: consistently diagnose access issues to --compress-program

* bootstrap.conf (gnulib_modules): Add findprog-in.
* src/sort.c: Include findprog.h.
(pipe_child): Look up the compress_program in $PATH and report errors
such as ENOENT or EACCES before invoking posix_spawnp.
This avoids inconsistency on systems that emulate posix_spawn through
fork/exec, as they would otherwise treat such a failure as a generic
failure and fail the sort, rather than continuing without compression.