If possible, use the posix_spawn call on Unix to execute commands.
The wrapper is derived from Ocamlnet's support library.
Merge branch 'posix_spawn' into perf-test

Conflicts:
	src/clib/OMakefile
	src/shell/omake_shell_sys_unix.ml
more instrumentation
performance: also avoid to create a fat thread when stdin/stdout is
a plain file (and not only when there is no redirection at all).
Exploit this in the standard build instruction for running ocamldep:
Instead of creating a pipe (which is really expensive) run ocamldep
and the postprocessor sequentially, and use a temp file to pass data
from one command to the next.
Merge branch 'posix_spawn' into perf-test

Conflicts:
	src/shell/omake_shell_job.ml
fix: calling+postprocessing ocamldep
Merge branch 'posix_spawn' into perf-test