[SV 63248] Ignore SIGPIPE

[SV 63248] Ignore SIGPIPE

Don't terminate when make's output is redirected to a pipe and the
reader exits early; e.g.:
  $ echo 'all:; sleep 2' | make -f- -j2 -O |:

This lets us unlink temporary files, and tell the user that make was
not able to write its output.
Reported by Frank Heckenbach <f.heckenbach@fh-soft.de>.

* src/main.c (main): Ignore SIGPIPE.
* src/posixos.c (osync_clear): Fix a memory leak.