[bug #60774] make hangs on fcntl lock when using -O and stdout is /dev/null

[Dmitry Goncharov]

Follow-up Comment #3, bug #60774 (project make):

i pushed a fix here
https://github.com/dgoncharov/make/tree/sv_60774_output_sync_deadlock.
This fix does the following.

Lock a temp file to synchronize output.

1. Lock a temp file, rather than stdout, to synchronize output.
Locking stdout prevents make from obtaining the lock when another process has
make's stdout locked. E.g make's stdout is redirected to /dev/null and another
process has /dev/null locked.

2. Pass the file descriptor of this lock file to children through env in
MAKEFLAGS' sync-mutex.

3. Check that a file can be locked before locking it.  With a temp file this
check in acquire_semaphore is not supposed to fail. It won't hurt though.

4. Print a warning message when cannot synchronize output.

Tested this fix on linux. Will be good if anybody could test this fix on
windows.

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